368 MANUAL OF ZOOLOGY. 



The nervous system varies considerably in its develop- 

 ment. 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 aper- 

 tures. The true Molluscan type (fig. 196), however, of ner- 

 vous 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-cesophageal " or "cere- 

 bral" ganglion. A second is placed below the oesophagus, 

 and is termed the " infra-oesophageal " 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. In many of the higher Mol- 

 lusca, however, the cerebral and pedal ganglia are fused in an 

 oesophageal ring. 



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., Strombidce), the eyes are of a very high 

 type of organisation. In the Lamellibranchiata 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 ("otocysts") 

 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 may be distinct, 

 or 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." 

 There is generally a distinct metamorphosis in development. 



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 



