MOLLUSCS. 321 



showed that this view of the homology was correct. It will be remembered that the 

 typical gill is innervated frona the visceral loop of the nervous system, and that near 

 its base is a patch of olfactory epithelium (see p. 250). Dissection of a Patella showed 

 that these prominences received their nerve supply from the visceral loop, and near 

 each was found an olfactory organ, thus making the homology complete and indisput- 

 able. The circular functional gill is therefore a superadded structure which has arisen 

 in a manner and from some cause not yet explained. 



Not all limpets have been shown to belong in this place, although future in- 

 vestigation may demonstrate that they should be classed here, a view which is 

 strengthened by the similarity in lingual dentition. Still, for 

 convenience, it will be well to consider the Acm^eid^ together 

 with this family, as in general appearance and in many important 

 points of structure they are closely similar. In these forms there 

 is a single cervical gill, while the circular marginal gill may be 

 either present or absent. The prominent genera are Acmcea, 

 Lottia., and Scurria. In Lepeta no gills are found. 



The shell in the limpets is conical, usually considei'ably de- 

 pressed, and is so characteristic as to have given rise to the adjec- 

 tives patelliform and limpet-like. The apex points forward, and 

 the internal horseshoe-shaped muscular impression, like that of the 

 Fissurellidse, is open in front. A large number of genera and 

 sub-genera have been made, the characters resting upon the res- ^^'^i^^^^n^ umpet. 

 piratory orggjis, shape and ornamentation of the shell, etc. 



As ordinarily found, the limpets are attached to some rock or other object by their 

 broad foot, and the strength with which they hold is astonishing. If the collector ap- 

 proach the animals suddenly, and with a quick motion slides them from their attach- 

 ment, he can get them easily, but if an incautious touch gives them warning of danger, 

 the shell will not infrequently break before the animal loosens its hold. The strange 

 story is told that each limpet has its own abiding place. At the time of high tide he 

 wanders off to find pastures of algee suitable for his palate, but as it ebbs he returns to 

 his chosen spot, and at low tide clings fast to the same spot on the rock which he left 

 a few hours before. 



On the European shores, limpets play an important part in the diet of the people 

 living near the shores, but on our coasts, except a very few used as bait, they have no 

 economic importance. Why it is that our people neglect so many articles of food it is 

 impossible to say. In the northern states, shrimps, limpets, periwinkles, mussels, etc., 

 are scarcely touched ; yet the sea teems with them, and everyone who has tried them 

 bears witness to their palatability. In the case of the limpets it may be that the com- 

 parative scarcity on our coasts may be the cause of their neglect. 



Acmcea testudinalis, the most common limpet on our northern coasts, 



belongs to the family Acmfeidse mentioned above. Its position here is 



extremely doubtful, and aside from the fact that its branchial system has 



never been studied, our only excuse for mentioning it here is to treat of 



all the limpets together. This species is very variable in color, but is 



^^'^' ^aiiTm"^^"" usually variously mottled with brown, pale green, and white. Like most 



of the limpets it lives between tide-marks. Acmcea alveus is a variety of 



the foregoing, but, from the habit that it has of living on eel-grass, it has acquired a 



narrower shell than the typical form. 



VOL. I. — 21 



