Chap, ix.] FORMS OF SHELLS. 311 



shells, it may be rudimentary, or absent. The operculum 

 is not represented in the Lamellibranchs. In a few cases 

 (e.g. the Lamellibranchiate Aspergillum) the protective 

 function is not assumed by the shell, which may be 

 quite small, but by the deposit of calcareous matter 

 on the siphon- shaped prolongations of the mantle. 

 (See page 80.) 



Among the Gastropoda the shell is often greatly 

 reduced, as in the slug, or completely lost, as in Doris 

 and other Nudibranehs, where the integument is, 

 however, richly supplied with calcareous spicules, and 

 in Oncidium, where the integument is thick and 

 leathery. One large division of Pteropods are 

 without any shell, and in the thecosomatous forms it 

 is always thin and glassy. 



The internal skeleton of Gastropods consists 

 merely of one or two pairs of cartilaginous plates, 

 which are found in the region of the pharynx. It is 

 better developed in the Cephalopoda, where it is 

 represented by a median cephalic cartilage, which 

 is pierced in the middle line by the resophagus, 

 and is produced on either side into plate-like supports 

 for the eye and ear; in some cases the orbit is 

 completely surrounded by cartilaginous pieces. In 

 the Dibranchiata the muscles which move the fins, 

 when those organs are developed, are inserted into 

 special cartilages, and other cartilaginous pieces are 

 more irregularly developed at the base of the funnel, 

 on the dorsal surface, or in the neck. 



The Brachiopoda have a hard external shell, 

 which, though it consists of two valves, is not to be 

 compared with that of the lamellibranchiate mollusc, 

 for their valves are not right and left, but dorsal and 

 ventral in position ; they may be subequal, as in 

 Lingula, or the ventral may be prolonged into the beak- 

 shaped free end, which has gained for these animals 

 their familiar name of " lamp-shell ; " in the latter 



