24 LECTURES ON MOLLUSCA. 



mantle. In the Nudibranclis , they form ornamental excrescences, 

 more or less diffused over the body. The sexes are always united. 



In all the water shell-fish, the animal after birth undergoes a meta- 

 morphosis, as in the insect tribe, before it assumes its normal condi- 

 tion ; but in the intermediate tribe of SNAILS, the creature is born into 

 its proper shape. The sexes are united, as in the Opisthobranchs. 



The NUCLEOBRANCHS have the gills in a tuft at the lower part of the 

 back, sometimes protected by a shell. They do not crawl like true 

 Gasteropods, but are an aberrant group passing over to the Pteropods. 

 They swim in the open sea ; and while they devour the jelly-fish, are 

 themselves the prey of true fishes and cuttles. 



ORDER PECTINIBRANCHIATA. (Comb-gilled Crawlers.) 



All these creatures have a spiral body, guarded by a shell. When 

 they walk about, the liver and other viscera remain in the upper por- 

 tion of the shell : but a large fleshy foot is protruded, on which the 

 animal crawls ; as also the head, with a distinct neck. On the head 

 are a pair of tentacles, (commonly called " horns," from their similar- 

 ity of position with the cow's horns,) which are extremely delicate 

 organs of sense. The eyes are on these, or at their base ; or, some- 

 times, on little eye-stalks near. In front is the snout, which is either 

 short, as in the periwinkles, or produced into a long trunk, as in the 

 carrion-feeding Strombs. Sometimes it appears very Itiort and inno- 

 cent ; but really it has swallowed, and can at any moment dart out, 

 an enormous proboscis, armed with powerful rows of teeth. The bot- 

 tom of the shell is in reality its front ; for there the animal breathes ; 

 there being either a pipe or a hole to let the water-current in to the 

 gills. The alimentary canal is doubled back over itself, terminating 

 near the gills, so as to be able to act, when the creature is at rest in 

 his shell. There are seldom any differences observable in the shells 

 of the two sexes. The intromittent organ is near the head, and gene- 

 rally very long ; varying considerably in shape in the different genera. 

 At the end of the foot is a horny operculum or toe nail ; which is drawn 

 in last of all into the shell, and serves to close its aperture, like a 

 trap -door. 



Remembering that the shell is part and parcel of the living animal 

 a secretion from its muscular skin or mantle of truly organized struc- 

 ture, though not endowed with feeling ; we shall naturally expect to 

 find differences in the shell corresponding with those in the sentient 

 inhabitant. This is generally, but not always, the case. Lamarck 

 thought that all creatures with a round-mouthed shell were herbivor- 

 ous, and all those with a notched mouth carnivorous ; but now it is 

 known that some round-mouthed groups are very fierce, as Natica and 

 Scalaria, while some that were thought predacious, as Cerithium, are 

 vegetarians. In Melania and lo, Bulimus and Achatina, we have both 

 forms of shell in one family. So Clark imagined that all creatures 

 with many-whirled opercula were hermaphrodites ; all with tew whirls 

 unisexual. But the hermaphrodite Nerites have few whirls; while 

 Modulus among the Periwinkles, and Cerithidea among the Cerites, 

 differ trom the other members of their unisexual families in having 



