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, Bidla, 
Accra, Aplustrum, Ringicvda, 
Gastropteron,Philine, Doridium, 
Aplysia, Lobiger, Pleuro- 
branchus, 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 contents, Scaphander bolts 
Sijnapta digitata, with parasitic Entoconclia, and middle 
PORTION ENLARGED. 
