356 MARINE INVERTEBRATES 
enemies when the animal retires within its fortress. They are 
all marine except a very few families, which, supposedly of marine 
derivation, have become terrestrial in habit. 
The prosobranchs are further subdivided into suborders accord- 
ing to certain peculiarities of the heart and breathing-organs. 
There is a group of these prosobranchs which gives evidence of 
an inferior degree of that visceral torsion which is always found 
in the gasteropods. In this group, or suborder, the heart has two 
auricles, and there is a pair of gills instead of only a single one. 
Other internal organs are paired just as they were represented to 
be in the schematic mollusk. This group also seems to show its 
primitive character in the want of a proboscis and a siphon, or, 
in some families, by having the ventricle of the heart traversed 
by the intestinal canal, just as in the lower class of mollusks, 
which includes the clams and oysters. For the most part the 
shells of this group are not typically spiral, but are patelliform, 
shield-like coverings, with only a suggestion of a spiral form at 
the very tip of the apex. This group of primitive prosobranchs 
is included in the following suborder: 
SUBORDER DIATOCARDIA 
This suborder is named from the presence of two auricles in 
the heart. 
FAMILY ACMEIDE 
-The first family to be noted is the Acmwide. Its principal 
genus, Acmea, is well represented on both the east and the west 
coast of the United States. 
Genus Aemea 
A. testudinalis. This species is found in vast numbers all along the 
New England shore, clinging to the rocks between tides. They are usu- 
ally called limpets. The shell is solid, conical, with an oval outline, and 
with no trace of a spiral form in the adult. When the shell is thor- 
oughly cleaned, it generally presents a mottled coloration of pale green, 
brown, and white. Inside it is white and nacreous, with a large brown 
