ORDER I. ACEPHALA TESTACEA. THE BIVALVES. 239 
and others, again, sojourn constantly near low-water mark. But by far 
the greater number dwell completely beyond the limits of the flood oscil- 
lations, at various distances from the surface, to a depth of five hundred 
feet and upwards. 
The sea-snails are either predaceous or herbivorous; the former bore 
through the shells of the sedentary mussels with their rasp-like tongue, 
or feast upon the dead animals which chance brings in their way. They 
seem to have very acute olfactory organs, for animal substances let down 
in a net to the bottom often draw thousands together in one night. In 
their turn, they serve as food to many other inhabitants of the ocean; but 
their deadliest enemies are the sea-stars, that not only swallow the young 
fry, but also seize with their long arms the full-grown Gasteropods, and 
clasp them in a murderous embrace. 
CLASS IV. ACEPHALES. 
The mollusks of this class owe their scientific name to the circumstance 
that they have no apparent head, the word being derived from the Greck 
a, no, and kephalee, head. Their mouth is concealed between the folds 
of their cloak, which latter is doubled in two, and encloses the body as a 
book is enclosed between its covers. A calcareous bivalve shell 
some- 
times multivalve—covers the cloak. The brain is situated over the 
mouth, which is destitute of teeth, and can seize only such objects as the 
water floats into it. The class is divided into two orders — the Testaceous 
acephales, and the Shell-less. The first order is by far the most numerous, 
as all bivalves, and nearly all multivalves, belong to it. 
ORDER I. ACEPHALA TESTACEA. THE BIVALVES. 
The Testacea are distinguished from the preceding mollusks by a more 
simple organization. The Gasteropod marches along by the aid of its 
powerful foot, and can thrust from its shell a well-developed head, while 
the Bivalve has neither foot nor head. Many of the bivalves, however, 
have eyes, or ocular spots, which enable them to distinguish light from 
darkness ; and some even possess auditory organs. 
When danger menaces the Sea-snail, it withdraws its head, and closes 
the entrance of its hermitage with a lid; but the bivalve shuts its folding- 
doors when it wishes to avoid a disagreeable intruder. A strong elastic 
ligament connects the two valves, and opens them wide as soon as the 
