Some of the molluscs like octopuses, squids, and cuttlefish (the cephalopods) are most adapted predators. The evolution of large brains in them takes place due to predatory lifestyles. They have complex sensory structures, rapid locomotion, grasping, tectacles, and tearing mouthparts. But cephalopods are not present in large number. Once their number of species was nine thousand. Now this class includes only about 550 species. Zoologists can explain the reason of their decline. There can be two reasons of their decline:
The vertebrates appeared in prehistoric seas. Sonic vertebrates acquired active, predatory lifestyles. It may be possible that vertebrates have outcompeted cephalopods.
The cephalopods may be declined simply because of random evolutionary events. This is not the case for all molluscs. Overall, this is very successful group. The molluscs have twice the number of vertebrates. Majority of the nearly 100.000 living species of molluscs belongs to two classes:
(a) Gastropoda: the snails and slugs
(b)Bivalvia: the clams and their close relatives.
Molluscs are triploblastic. They are the first animals that possess a coelom. But the coelom of molluscs is only a small cavity. This is called pericardial cavity. It surrounds art and gonads. A coelom is a body cavity that arises in mesoderm and is lined by a sheet of mesoderm called peritoneum.
RELATIONSHIPS TO OTHER ANIMALS
Molluscs are protostomes. In protosotme, blastopore form mouth. Therefore, there are great similarities in the embryological development of the molluscs and other proto tomes like annelids (segmented worms). These similarities are:
1. Both have similar trochophore larvae.
2. Certain adult structures of Molluscs and annelids are similar. For example, the excretory organs and their duct systems are similar in them.
Most zoologists accept the protostomes relations of the molluscs. But the relationship between the members of this phylum and other protostomes is distant. Therefore, the ancestral evolutionary pathways are speculative.
ORIGIN OF THE COELOM
There are a number of hypotheses about the origin of the coelom. These hypotheses relationships among triploblastic phyla.
1. Schizocoel hypothesis: (Gr. schizen. to split + koilos, hollow)
In this case, coelom is formed by the splitting of mesoderm. It is found in all protostomes. Mesoderm fills the area between ectoderm and endoderm. Coelom is formed by the splitting of this mesoderm. It indicates that mesodermally derived tissues are formed before the coelom formation. It suggests that a triploblastic acoelomate (flatworm) body form is the ancestor of the coelomate body form.
2. Enterocoel hypothesis: (Gr. enteron. gut + koilos, hollow)
It suggests that the coelom have arisen as out pocketing of a primitive gut. This pattern of coelom formation is present in deuterostomes. The animals in which blastopore forms anus and mouth formed as a secondary opening are called deutrostomes. This hypothesis suggests that mesoderm and the coelom are formed from the gut of a diploblastic animal. It means that the mesoderm fills the body cavity of a coelomate yriimal and triploblastic acoelomate body was formed. Therefore, acoelomate body form was secondarily derived from the coelomate body form.
Unfortunately, zoologists do not know which hypothesis is correct. Some zoologists believe that the coelom was formed more than once in different evolutionary lineages. Therefore, more than one explanation can be correct.