Phylum Mollusca and Its Classification | Coelomates | Zoology

The phylum Mollusca contains over 100,000 species with a variety of body forms and lifestyles. In mollusks, the coelom is reduced and limited to the region around the heart. The Mollusk body first appeared during the Cambrian Period.

All mollusks have:

1. A visceral mass containing internal organs, including the digestive tract, paired kidneys, and reproductive organs.

2. A mantle that surrounds but does not cover entirely the visceral mass and secretes a shell (if one is present). The mantle also contributes to formation of gills or lungs.

3. A head/foot region containing sensory organs and a muscular structure (foot) used for locomotion. The foot is a muscular structure used for locomotion, attachment to a substrate, food capture, or a combination of functions.

4. A radula is an organ that bears many rows of teeth and is used for grazing on food.

5. The nervous system consists of several ganglia connected by nerve cords.

Most mollusks have an open circulatory system- a heart that pumps hemolymph through vessels into a hemocoel. Blood diffuses back into the heart and is pumped out to the body again. Some mollusks are slow moving, and have no heads, while others are active predators that have a head and sense organs.

1. Class Polyplacophora:

Chitons, are in the taxonomic class Polyplacophora. They have a shell consisting of eight overlapping plates.

A ventral muscular foot is used for creeping along the substrate, or for clinging to rocks. A chiton feeds by scraping algae and other plant food from rocks with its well-developed radula.

2. Class Gastropoda:

The class Gastropoda includes snails, terrestrial slugs, whelks, conchs, periwinkles, sea hares, and sea slugs. Most gastropods are marine, although some are freshwater and terrestrial forms. Many gastropods are herbivores that use their radula to scrape food from surfaces. Carnivorous gastropods use their radula to bore through a surface, such as a bivalve shell, to obtain food.

Most gastropods have a well-developed head with eyes and tentacles projecting from a coiled shell that protects the visceral mass. The coiled shells of gastropods are often quite commonly found as fossils. One genus, Turetella, occurs in such quantities in a type of rock that the rock is known as “Turetella agate”. However, not all gastropods have shells, the nudibranchs (sea slugs) and terrestrial slugs lack shells.

In aquatic gastropods, gills are found in the mantle cavity; in terrestrial gastropods, the mantle is richly supplied with blood vessels and functions as a lung when air is moved in and out through respiratory pores. In 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 lubricating mucus.

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.

3. 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 cephalisation. 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 towards 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.

Fossil Record of Bivalves:

Since they have hard shells, the fossil record of this class is remarkably good. 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.

During the Mesozoic era, bivalves became more abundant and important parts of reefs. They would remain important parts of the marine fauna throughout the Mesozoic. These bivalves, specifically the rudistids, began 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.

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.

4. 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 thus propelling them with a form of jet propulsion. Surrounding their head are tentacles with suckers that can grasp prey and deliver it to a powerful beak/mouth. Cephalopods in general have well-developed sense organs, including focusing camera-type eyes.

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 100 eggs.

Evolutionary History 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 excellent fossil record for the group. The Ordovician period saw the evolution and spread of coiled, swimming cephalopods. This group, the nautiloids, resembles somewhat their living distant relatives the chambered nautilus and squids.

During the Devonian period Cephalopods ammonoid group known as the goniatites appeared.

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.

The cephalopods recovered from the extinction of the goniatites at the close of the Paleozoic era and developed a remarkably similar group, the ceratites. However, the Ceratites also went extinct during the middle Mesozoic era.

During the Jurassic, the cephalopods once again produced a new coil. The belemnites were straight-shelled cephalopods with elaborate suture patterns. Both ammonites and belemnites survived the Jurassic extinctions and flourished during the Cretaceous period.

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.