COVERED-GILLED SUBGROUP 



3331 



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, Gastrop- 



teron, Philine, Doridium, Aplysia, Lobiger, 



Pleurobranchus, Umbrella, and the aberrant 



Siphonaria. Actceon, 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 



I.ARVA OF Entoconcha (much magnified). 



Acera bullata (twice natural size). 



contents, Scaphander bolts its prey whole, shell and all. The shell is then broken 

 up by the aid of a very remarkable gizzard, and the soft parts digested. This giz- 

 zard consists of three strong calcareous plates of different size and form, which 

 grind against one another by powerful muscles. The shells of Scaphander are 

 mostly thin, spirally striated, oblong, convolute, without any visible spire, having 

 the aperture narrow behind, and much wider in front. The known species are not 

 numerous, and occur chiefly in seas of the Northern Hemisphere, but a few have 

 been described from other parts of the world. S. mundus was obtained at the Aru 



