352 MARINE INVERTEBRATES 



hares." They have a mantle so greatly developed that it 

 actually covers the shell, arid its edges unite and fuse over the 

 top. The shell, in consequence, having almost ceased to be of 

 use as a protection, has degenerated into a mere horny plate, 

 and has lost all resemblance to the ordinary gasteropod shell. 

 Having practically lost its protective office and become a mere 

 internal plate, it is quite probable that it will in time wholly 

 disappear. The gills of the sea-hares are concealed under a flap 

 of the mantle, their position being posterior to the heart. The 

 most conspicuous representative of this type of tectibranchs in 

 the United States is the following : 



GENUS Aplysia 



A. WUcoxil. This species appears at times in vast numbers in the 

 waters of Florida, until the sea may truly be said to be fairly alive with 

 them. They swim lazily with a waving motion of the parapodia. They 

 disappear as mysteriously as they come, and for months not a specimen 

 will be seen. There is a variety of this Floridian Aplysia which occurs 

 at Cape May, but no sea-hares are to be found north of that point. The 

 tropical Pacific furnishes an astonishing wealth of these creatures, 

 belonging to many genera and species, and among them are some of the 

 most beautiful of the invertebrate animals. 



SUBORDER NUDIBBANCHIATA 



The opisthobranchs are divided into two suborders, the second 

 of which is called NudibrancMata. The name is well chosen and 

 very suggestive, for it means " naked or exposed gills " ; but this 

 anatomical feature is only one of the peculiarities of this suborder. 

 The nudibranchs are commonly known as " sea-slugs " ; for, like 

 the land-sings, which are also true mollusks, they possess no 

 shell at all. That they at one time carried a shell is evident from 

 the fact that they are born with a rudimentary testaceous cover- 

 ing, which soon afterward disappears. 



A striking peculiarity of the nudibranchs is that the conven- 

 tional molluscan mantle is not usually apparent. Instead of 

 seeing the usual flaps or folds of the mantle which more or less 

 encircle mollusks, and which one seems to have a right to expect, 



