Question

Section 28.4

1. Describe the three key characteristics shared by all the major groups of mollusks.

2. Describe the following classes of mollusks. Give examples of each.

1. Bivalves:

2. Gastropods:

3. Cephalopods:

3. How do cephalopods differ from other mollusks in terms of their nervous system, reproduction, locomotion, and mode of obtaining food?

4. Describe the defining characteristics of annelids and distinguish between the two classes of annelids. Provide examples of each.

Answer 1

1.Key characteristics shared by all the major groups of mollusks:

• Unsegmented soft body with bilateral symmetry.
• Presence of an internal or external shell.
• A mantle which is a fold in the body wall that lines the shell.

2. (a) Bivalves:
The bivalves are a large class of molluscs, also known as pelecypods. They have a hard calcareous shell made of two parts or 'valves'. The soft parts are inside the shell. The shell is usually bilaterally symmetrical. There are over 30,000 species of bivalves, including the fossil species. Most bivalves are filter feeders, using their gills to capture particulate food such as phytoplankton from the water. The protobranchs feed in a different way, scraping detritus from the seabed, and this may be the original mode of feeding used by all bivalves before the gills became adapted for filter feeding.
Bivalves include clams, scallops, oysters, and mussels.

(b) Gastropods:
The Class Gastropoda includes the snails and slugs. Most gastropods have a single, usually spirally coiled shell into which the body can be withdrawn, but the shell is lost or reduced some important groups.They are by far the largest group of molluscs, with more than 62,000 described living species, and they comprise about 80% of living molluscs. Estimates of total extant species range from 40,000 to over 100,000, but there may be as many as 150,000 species.
Gastropods feed on very small things. Most of them scrape or brush particles from surfaces of rocks, seaweeds, animals that don't move, and other objects. For feeding, gastropods use a radula, a hard plate that has teeth.

(c)Cephalopods:
A cephalopod is any member of the molluscan class Cephalopoda such as a squid, octopus, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot.
The class now contains two, only distantly related, extant subclasses: Coleoidea, which includes octopuses, squid, and cuttlefish; and Nautiloidea, represented by Nautilus and Allonautilus.Cephalopods are marine predators; these carnivores eat fish, worms, crustaceans, and other mollusks. Some cephalopods include the octopus, squid, cuttlefish, nautilus, and ammonite (and other extinct animals).

3. Cephalopods differ from other mollusks.

Nervous system: Cephalopods have a more developed nervous system than other mollusks.The nervous system of cephalopods is the most complex of the invertebrates and their brain-to-body-mass ratio falls between that of endothermic and ectothermic vertebrates.

Reproduction:Cephalopods have separate sexes and spermatophores are transferred between males and females by modified tentacles. Eggs are large and yolk-rich, and embryonic development of cephalopods is different from that of all other mollusks. There is no larval form, just direct development into juveniles.

Locomotion: Cephalopods also have amorphous bodies and can move around very quickly than other mollusks.

Mode of obtaining food: This is the only class of mollusks in which the organisms are carnivores.

4. Characteristics of annelids:

• The Annelids are coelomate and triploblastic.
• They exhibit organ level organization.
• Their body is segmented.
• They respire through their body surface.
• Nephridia are the excretory organs.
• They have a well-developed circulatory and digestive system.
• Their body contains haemoglobin, which gives them a red colour.
• Regeneration is a very common characteristic of the Annelids.
• Setae help them in movement.
• Most of the Annelids are hermaphrodite, i.e., male and female organs are present in the same body. They reproduce both sexually and asexually. The others reproduce sexually.
• Eg., Earthworms, and leeches

There are three classes of annelids:

• Class Polychaeta: marine annelids;
• Class Oligochaeta: marine, freshwater and terrestrial annelids including earthworms;
• Class Hirudinea: marine, fresh water and terrestrial leeches.