MOLLUSCA: GENERAL CHARACTERS. 229 



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

 majority 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 Tunicate, 

 placed between the oral tentacles. Organs of hearing exist in 

 the more highly organised Mollusca, especially in the Gastero- 

 poda 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 Tunicota, are pro- 

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

 ample of non-sexual reproduction. The sexes are usually 

 distinct, 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 rudimentary 

 shell enclosed in the mantle, the Mollusc is said to be ' naked.' 

 The shell of the ' testaceous ' Mollusca is very closely related to 

 the respiratory organs ; ' indeed, it may be regarded as a pneu- 

 mosJceleton, being essentially a calcified portion of the mantle, 

 of which the breathing-organ is at most a specialised part. 



In its most reduced form the shell is only a hollow 



cone, or plate, protecting the breathing-organ and heart, as in 

 Limax, Testacella, and Carinaria. Its peculiar features always 

 relate to the condition of the breathing-organ, and in Terebra- 

 tula and Pelonaia it becomes identified with the gill. In the 

 Nudibranchs the vascular mantle performs, wholly or in part, 

 the respiratory office. In the Geplialopods the shell becomes 

 complicated by the addition of a distinct, internal, chambered 

 portion (phragmacone), which is properly a visceral skeleton.' 

 (Woodward.) In a great many of the Mollusca proper the 

 shell consists of but a single piece, and they are called * uni- 

 valves.' In many others the shell consists of two separate 

 plates, or 'valves,' and these are called 'bivalves.' In others, 



