Flatworms, Mollusks, Annelids - Biology LibreTexts
The phylum Mollusca is a diverse group of. invertebrates. Annelids are worms with. cylindrical, segmented bodies. Arthropods. are segmented. Phylum Arthropoda: Segmented Bodies with Segmented Appendages | Deuterostomes and Suggested evolutionary relationships of the Mollusca. Certain clams and annelids have the same type of larva, hinting at a possible evolutionary. The phyla Mollusca and Annelida belong to a clade called the They are distinct from the Ecdysozoa (nematodes and arthropods) based on evidence which has changed our views of the relationships among invertebrates.
These animals have a radula modified for scraping. A single pair of nephridia for excretion is present. This chiton from the class Polyplacophora has the eight-plated shell indicative of its class.
They are found in marine and freshwater habitats. As the name suggests, bivalves are enclosed in a pair of shells or valves that are hinged at the dorsal side. The body is flattened on the sides. They feed by filtering particles from water and a radula is absent.
They exchange gases using a pair of ctenidia, and excretion and osmoregulation are carried out by a pair of nephridia. In some species, the posterior edges of the mantle may fuse to form two siphons that inhale and exhale water. This property is commercially exploited to produce pearls.
Gastropods include shell-bearing species as well as species with a reduced shell. These animals are asymmetrical and usually present a coiled shell Figure Most gastropods bear a head with tentacles that support eyes.
A complex radula is used to scrape food particles from the substrate. The mantle cavity encloses the ctenidia as well as a pair of nephridia.
Cephalopods include shelled and reduced-shell groups. They display vivid coloration, typically seen in squids and octopuses, which is used for camouflage. The ability of some octopuses to rapidly adjust their colors to mimic a background pattern or to startle a predator is one of the more awe-inspiring feats of these animals. All animals in this class are predators and have beak-like jaws. All cephalopods have a well-developed nervous system, complex eyes, and a closed circulatory system.
The foot is lobed and developed into tentacles and a funnel, which is used for locomotion. Suckers are present on the tentacles in octopuses and squid. Ctenidia are enclosed in a large mantle cavity and are serviced by large blood vessels, each with its own heart. Cephalopods have separate sexes, and the females of some species care for the eggs for an extended period of time. Although the shell is much reduced and internal in squid and cuttlefish, and absent altogether in octopus, nautilus live inside a spiral, multi-chambered shell that is filled with gas or water to regulate buoyancy.
The a nautilus, b giant cuttlefish, c reef squid, and d blue-ring octopus are all members of the class Cephalopoda.
Tooth shells have a radula and a foot modified into tentacles, each with a bulbous end that catches and manipulates prey Figure Approximately 16, species have been described.
The phylum includes earthworms, polychaete worms, and leeches. Like mollusks, annelids exhibit protostomic development. Annelids are bilaterally symmetrical and have a worm-like appearance. Their particular segmented body plan results in repetition of internal and external features in each body segment. The evolutionary benefit of such a body plan is thought to be the capacity it allows for the evolution of independent modifications in different segments that perform different functions.
The overall body can then be divided into head, body, and tail. Physiological Processes of Annelida The skin of annelids is protected by a cuticle that is thinner than the cuticle of the ecdysozoans and does not need to be molted for growth. The chaetae are a defining character of annelids. Polychaete worms have paired, unjointed limbs called parapodia on each segment used for locomotion and breathing.
Beneath the cuticle there are two layers of muscle, one running around its circumference circular and one running the length of the worm longitudinal.
Annelids have a true coelom in which organs are distributed and bathed in coelomic fluid. Annelids possess a well-developed complete digestive system with specialized organs: Gas exchange occurs across the moist body surface.
Annelids have a well-developed nervous system with two ventral nerve cords and a nerve ring of fused ganglia present around the pharynx. In this schematic showing the basic anatomy of annelids, the digestive system is indicated in green, the nervous system is indicated in yellow, and the circulatory system is indicated in red. Annelids may be either monoecious with permanent gonads as in earthworms and leeches or dioecious with temporary or seasonal gonads as in polychaetes.
The chaetae of polychaetes are also arranged within fleshy, flat, paired appendages on each segment called parapodia. Terrestrial gastropod embryonic development does not go through a swimming larval stage, as is the case in aquatic gastropods. For terrestrial snails, their shell not only offers protection but also prevents desiccation drying out.
The muscular foot contracts in peristaltic waves from anterior to posterior causing secretion of a lubricating mucus. Top Image from Purves et al. Vintage Illustration from the Lycos Image Gallery. Terrestrial gastropods are hermaphroditic. In premating behavior, they meet and shoot calcareous darts into each other's body wall. Each inserts a penis into the vagina of the other, providing sperm for future fertilization of eggs.
Eggs are deposited in the soil and development proceeds without formation of a larval stage, a common theme in some terrestrial invertebrates. Hermaphroditism assures that any two animals that meet can mate, which is especially useful in slow-moving animals.
The Class Bivalvia The class Bivalvia consists of clams, oysters, mussels, and scallops. Members of this class have two-part shells that are hinged and closed by powerful muscles. The presence of shells in this group has yielded an impressive fossil record. The bivalves have no head, no radula, and little cephalizationas can be seen in Figure 5. Clams use their hatchet-shaped foot for burrowing; mussels use it to produce threads to attach to objects.
Scallops can both burrow or swim. A rapid closing and opening of their two valves releases water in spurts. The bivalve shell is secreted by the mantle. The shell is composed of protein and calcium carbonate with an inner layer of pearl. Pearls form as layers of shell-forming material deposited about a foreign particle lodged between the mantle and the shell. A compressed muscular foot projects down from shell. By expanding the tip, the foot pulls the body after it.
Beating cilia of the gills cause water to enter the mantle cavity by way of the incurrent siphon and to exit by way of the excurrent siphon. While cilia of gills move water through the mantle cavity, gills also capture particles in water and move them toward the mouth. From the mouth food goes to the stomach, then to the intestine, which passes through the heart and ends at the anus.
Bivalves, like other mollusks, have an open circulatory system. Their nervous system consists of three pairs of ganglia. Two excretory kidneys below the heart remove ammonia waste from the pericardial cavity into the mantle cavity, from which it will leave the body. Sexes in class Bivalvia are separate. The gonad is located around the coils of the intestine. Certain clams and annelids have the same type of larva, hinting at a possible evolutionary relationship between the two groups.
Anatomy of a bivalve. Fossil Record of Bivalves Since they have hard shells, the fossil record of this class is remarkably good.
- 15.4: Mollusks and Annelids
Hard shells or hard parts are one of the features that make an organism a better candidate to become a fossil. Gastropods, another class of the phylum Mollusca, also become more prevalent in the Ordovician seas.
Ordovician deposits yield snails, as well as large, sedentary gastropods such as Maclurites, shown in Figure 6. Maclurites magnus, a large ancient gastropod. This image is from http: During the Mesozoic era, bivalves became more abundant and important parts of reefs. They would would remain important parts of the marine fauna throughout the Mesozoic. These bivalves, specifically the rudistids Figure 7began to play a larger role in reef formation. Rudistid reefs are so named because the rudistid bivalves were the dominant reef-forming organisms.
Biodiversity was reduced by mass extinctions at the end of the Triassic and Jurassic periods of the mesozoic era. Two bivalves from the Jurassic of Germany. The bivalves recovered from the Jurassic extinctions and again became major reef-formers in the numerous shallow marginal seas that encroached onto the continents during the Cretaceous, as shown by Figure 8.
Reconstruction of a Cretaceous seafloor. Note the large ammonite on the right, the belemnites in the center, and the gastropods and bivalves on the seafloor. In this view we see the top of one of the shells. The lower shell was usually quite different in shape.
Mollusks, Annelids, and Arthropods by Vidya Venkatesh on Prezi
The Class Cephalopoda The class Cephalopoda literally "head-footed" includes squids, cuttlefish, octopuses, and nautiluses and extinct relatives, the goniatites, ammonoids, and ammonites. The presence of a shell in many representatives of this class has yielded an impressive fossil record. Squids and octopuses can squeeze water from their mantle cavity out through a funnel shown in Figure 9thus propelling them with a form of jet propulsion.
Cephalopods in general have well-developed sense organs, including focusing camera-type eyes.
Mollusks and Annelids - Biology LibreTexts
Most cephalopods, especially octopuses, have well-developed brains and show a capacity for learning. Nautiluses are enclosed in shells, squids have a shell that is reduced and internal, while octopuses lack a shell. Squids and octopuses possess ink sacs from which they squirt a cloud of ink, as a means of escaping predators.
Squids possess a vestigial skeleton under the mantle, called the pen, which surrounds the visceral mass. A squid has three hearts, one pumps blood to internal organs; two pump blood to the gills in the mantle cavity. Gonads make up a large portion of the visceral mass. Cephalopds have separate sexes. Spermatophores contain sperm, which the male passes to the female mantle cavity by way of one of his tentacles.
After fertilization, eggs are attached to the substratum in strings containing up to eggs. Anatomy of a cephalopod. Smithsonian Photo by Jessie Cohen. Fossil Record of Cephalopods During much of their evolutionary history cephalopods possessed a hard shell. Their abundance, the presence of a shell, and the environments they lived in led to an excellen fossil record for the group. The Ordovician period saw the exolution and spread of coiled, swimming cephalopods.
This group, the nautiloids shown in Figure 10resembles somewhat their living distant relatives the chambered nautilus and squids. Lituites littuus, an odd nautiloid fossil from the Ordovician of China.
During the Devonian period Cephalopods ammonoid group known as the goniatites appeared. These coiled, chambered nautiloids, some of which are shown in Figure 11, left a great many fossils, some of which are quite aesthetically appealing. In this specimen the surrounding matrix has been cut away and the fossils cut to reveal the inner chambers.
The ammonoids underwent three separate diversifications from a nautiloid-like stock. In each case the fold pattern of sutures became more complex. These sutire patterns are fantastic characters for identifying species, making ammonoids excellent index fossils. The first of these occurrences was the goniatites, a group that ranged from the Devonian to the Permian.
The ceratites are a Triassic group, while the last group, the ammonites ranged from the Triassic to the Cretaceous. Ammonoids finally went extinct in the great end-of-the-Cretaceous extinction.
Nautiloids are represented today by the Nautilus. Differences between the groups are shown in Figure Comparison of the suture patterns of the goniatites, ceratites, and ammonites. Animated GIF image of an anmmonoid, showing the relationship of the fossil to the presumed location of the living animal. The cephalopods recovered from the extinction of the goniatites at the close of the Paleozoic era and developed a remarkably similar group, the ceratites. These coiled, chambered animals, such as the one shown in Figure 12, had slightly more complex sutures than did the goniatites.
However, the Ceratites also went extinct during the middle Mesozoic era. Ceratites nodosus from the Triassic of Saverne, France. During the Jurassic, the cephalopods once again produced a new coiled, chambered form, the ammonites, shown in Figure 13 and Suture patterns of these forms were even more elaborate than those found in the Triassic ceratites. The belemnites were straight-shelled cephalopods with elaborate suture patterns.
Both ammonites and belemnites survived the Jurassic extinctions and flourished during the Cretaceous period. Dactyliceras commune, an ammonite from the early Jurassic of Withby, Yorkshire, England. A complete specimen is shown on the left, while a sectioned specimen is on the right. Ammonites continued their dominance, as did their relatives the straight-shelled belemnites. Modern teleost fish appeared during the Cretaceous and may have competed for the same prey as the ammonites.
The teleost fish were apparently stronger and swifter swimmers than the fish of the Jurassic. Some paleontologists speculate that the extinction of ichthyosaurs during the Cretaceous may have been hastened by the rise of these new faster fish that would have been difficult for the ichthyosaurs to catch and eat.
Baculites, a genus of straight-shelled cephalopods, was particularly abundant in the Cretaceous seas. Note the elaborate suture patterns in the fossil specimen below. Common fossils in the Cretaceous rocks, the cephalopods were major victims along with the gastropod group the rudistids of the terminal Cretaceous extinction event.
Squid, octopus, and the chambered nautilus are the remnants of this once flourishing group of molluscs. The specimen is 2. Baculites was a straight-shelled cephalopod, about two feet in length, that presumably scavenged the bottom in search of food.
Here, a small cephalopod becomes dinner image from http: Notice the extremely intricate suturing between septa. The development of segmented bodies allowed the formation of specialized functions in different segments.
Annelids have an enlarged coelom to accommodate more complex internal organs. The well-developed, fluid-filled coelom and the tough integument act as a hydrostatic skeleton. There are about 12, marine, freshwater, and terrestrial species usually divided into three taxonomic classes. Similarities of larval forms to Mollusks suggest annelids share an common ancestral group. Annelids have a closed circulatory system with blood vessels running the length of the body and branching into every segment.
Closed circulatory systems are more efficient than open ones for moving materials within a body. The annelid nervous system consists of a brain connected to a ventral solid nerve cord, with a ganglion in each segment. Annelids have a complete digestive system that include a pharynx, stomach, intestine, and accessory glands.
Excretory nephridia in each segment collect waste material from coelom and excrete it through the body wall. Note the presence of a coelom. Images from Purves et al. Classification of the Annelida The Class Polychaeta Most annelids belonging to the taxonomic class Polychaeta are marine and possess parapodia and setae. Parapodia are paddlelike appendages used in swimming that also serve as respiratory organs.Invertebrate Animals - Educational Video for Kids
Setae are bristles, attached to parapodia, that help anchor polychaetes to their substratum and also help them move. Clam worms, such as Nereis, are active predators. Many have well-developed cephalization, with a head having well-developed jaws, eyes, and other sense organs.
Sedentary filter feeders possess tentacles with cilia to create water currents and to select food particles. Only during breeding do polychaetes have reproductive organs. Polychaet zygotes develop into a type of larva similar to that produced by marine clams. The Class Oligochaeta The class Oligochaeta includes earthworms, that tend to have their few setae protruding in clusters directly from their body.
Earthworms have poorly developed heads or parapodia. Locomotion is by coordinated movement of the body muscles and assistance of their setae. When longitudinal muscles contract, segments bulge and setae protrude and anchor into the soil.
Flatworms, Mollusks, Annelids
Circular muscles contract, causing the worm to lengthen, setae are withdrawn and the segment moves forward. Earthworms reside in moist soil where a moist body wall facilitates gas exchange.
Earthworms are scavengers that extract organic remains from the soil they eat. A muscular pharynx draws food into the mouth. Ingested food is stored in a crop and ground up in a muscular gizzard. The dorsal surface of the intestine is expanded into a typhlosole that allows more surface area for digestion. External segments correspond to internal septa walls separating each body segment. The earthworm excretory system has coiled nephridia tubules in each segment with two openings: Between the two openings, the coiled nephridia tubule allows removal of waste materials from blood vessels.
Red blood is moved anteriorly by a dorsal blood vessel and pumped by five pairs of hearts sometimes referred to as aortic arches to a ventral vessel. Earthworms are hermaphroditic, having both testes with seminal vesicles, and ovaries with seminal receptacles. Mating involves the worms lying parallel to each other facing opposite directions and exchanging sperm.
Each worm possesses a clitellum that then secretes a mucus, protecting sperm and eggs from drying out. Embryonic development lacks a larval stage. The Class Hirudinea The class Hirudinea includes leeches.
Most are freshwater, but a few are marine or terrestrial. Each body ring has several transverse grooves. Leeches possess a small anterior sucker around the mouth and a larger posterior sucker. Although some are free-living predators, most are fluid feeders. Bloodsuckers keep blood from coagulating by hirudin, an anticoagulant in their saliva. Leeches were commonly used in early medicine to "bleed" the patient. Segmented Bodies with Segmented Appendages Back to Top The phylum Arthropoda contains animals with segmented appendages on their body segments.
Arthropods occupy every habitat, and are in many respects the most successful animal group on Earth.