GASTROPODS. 



361 



Nudibranchs in regard to the position of the rhinophores, the lateral expansions 

 of the body, and the radula. Nearly all the Tectibranchs possess a shell which 

 sometimes is wholly concealed beneath the mantle, sometimes partly covered, 

 and, in many instances, wholly external. Some are carnivorous, others entirely 

 phytophagous or vegetable- 

 feeders; they mostly progress 

 like an ordinary snail by means 

 of the foot, but a few of them 

 are free swimmers. The ova 

 are generally deposited in long 

 ribbons more or less coiled, and 

 the embryos are provided with 

 an operculated spiral shell. The 

 Tectibranchs are classified in a 

 number of families, separated 

 from one another by differences 

 in the general conformation of 

 the animals, the shell, and the 

 radula. The following are the 

 more important genera: — 

 Actceon, Scaphander, Bulla, 

 Acera, Aplustrum, Ringicula, 

 Gastropteron, Philine, Doridium, 

 Aplysia, Lobiger, Pleuro- 

 branchus, Umbrella, and the 

 aberrant Siphonaria. Actwon, 

 of which about thirty species 

 are known, is found in all seas. 

 The animals are provided with 

 a sort of head-shield, which is 

 notched behind, and have the 

 sessile eyes in front of the notch. 

 They all are furnished with an 

 ovate, often spirally punctate 

 shell, which in some cases is 

 prettily striped or spotted, and is 

 capable of receiving the entire 

 animal when contracted. In 

 Scaphander the animal is too 

 large to be wholly contained 

 within the shell, and has 

 an enormous head -disc ; and the sides of the body are produced into lobes or 

 epipodia, which can be partly reflexed over the shell. It has no visible eyes, 

 as these would scarcely be of service to an animal which burrows into mud and 

 sand in search of its food. Unlike many of the carnivorous Gastropods, which 

 bore a hole in the victim's shell, and suck out the contents, Scaphander bolts 



Synapta digitata, with parasitic Entoconcha, and middle 



PORTION ENLARGED. 



