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. Tins group of primitive prosobranchs 

 is included in the following suborder : 



SUBORDER DIATOCABDIA 



This suborder is named from the presence of two auricles in 

 the heart. 



FAMILY ACILEIDJE 



The first family to be noted is the Acmceidce. Its principal 

 genus, Acmcea, is well represented on both the east and the west 

 coast of the United States. 



GENUS Acmcea 



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 



