MOLLUSCA: GENERAL CHARACTERS. 255 



The nervous system varies considerably in its development. 

 In the Polyzoa, Tunicata, and Brachiopoda which collectively 

 constitute the Molluscoida the nervous system consists of a 

 single ganglion, or of a principal pair with accessory ganglia, 

 placed between the oral and anal apertures, or on the ventral 

 surface of the body. The true Molluscan type (fig. 93), how- 

 ever, of nervous system is constituted by the presence of three 

 pairs of ganglia, connected with one another by commissures, 

 but distributed in a characteristically scattered manner (hetero- 

 gangliate type). One of these ganglia is situated above the 

 oesophagus, and is called the "supra-oesophageal" or "cerebral" 

 ganglion. A second is placed below the oesophagus, and is 

 termed the " infra-cesophageal " or "pedal" ganglion (from its 

 supplying the nerves to the " foot "). The third pair is the 

 most persistent, and is termed the "branchial" or " parieto- 

 splanchnic " ganglion. 



Organs of sight exist in some of the lower, and in the majo- 

 rity of the higher, Mollusca. In the Cephalopoda, and in some 

 of the Gasteropoda (e.g. Strombidcz}, the eyes are of a very high 

 type of organisation. In the Lamettibranchiata the adults are 

 either destitute of organs of vision, or possess numerous simple 

 eyes ("ocelli") placed along the margins of the mantle-lobes. 

 Similar ocelli are also found in some of the Tunicata, placed 

 between the oral tentacles. Organs of hearing exist in the 

 more highly organised Mollusca, especially in the Gasteropoda 

 and Cephalopoda, and supposed olfactory organs occur in some 

 of the latter. 



Reproduction amongst the Mollusca is almost invariably 

 sexual, but it is by continuous gemmation that the colonies of 

 the Polyzoa, and the social and compound Tunicata, are pro- 

 duced, and the " statoblasts " of the former offer a good ex- 

 ample of non-sexual reproduction. The sexes are usually dis- 

 tinct, but are in many cases united in the same individual. In 

 many forms the ova are arranged in rows, so as to form a strap 

 or ribbon shaped structure, termed the " nidamental ribbon." 



As implied by their scientific name, the Mollusca are mostly 

 soft-bodied animals, but their popular name of " Shell-fish " 

 expresses the fact, that the presence of a shell, protecting the 

 soft body, is likewise a very characteristic feature in the sub- 

 kingdom. At the same time, a shell is not universally present, 

 and many of the Mollusca are either permanently naked, or 

 possess nothing that would be ordinarily looked upon as a 

 shell. When there is either no shell at all, or merely a rudi- 

 mentary shell enclosed in the mantle, the Mollusc is said to be 

 "naked." The shell of the "testaceous" Mollusca is very 



