352 MARINE INVERTEBRATES 
hares.” They have a mantle so greatly developed that it 
actually covers the shell, and 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. Witlcoxii. 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 NUDIBRANCHIATA 
The opisthobranchs are divided into two suborders, the second 
of which is called Nudibranchiata. 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-slugs, 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, 
