GASTEROPODS 353 
they are entirely absent, and the body of the sea-slug assumes in 
consequence a worm-like appearance. Other notable features of 
the nudibranchs are the great number of tentacular processes that 
usually project from the dorsal region of the animal, and, in many 
of the genera, an entire absence of gills. When the gills are 
present, as is the case in several of the nudibranch families, they 
are not placed along the side of the animal, where one would 
naturally look for them, but are arranged in the form of a 
rosette of plume-like processes situated in the posterior dorsal 
region, or, in other words, on the animal’s back. 
Nudibranchs are commonly to be found all along the Atlantic 
and Pacifie coasts of North America, and more especially in the 
colder waters north of Cape Cod. They are essentially littoral, 
and live upon alge in shallow water, upon eel-grass in sheltered 
places, and in tide-pools, where there is more or less vegetation. 
They crawl about the fronds of alge, or swim, foot upward, with 
a gentle and undulating motion, or, when caught between tides, 
may be seen clinging to the under surface of rocks. Protective 
resemblances have been so remarkably developed in the nudi- 
branchs that’ they are not always easy to discover; indeed, one 
may actually be looking at one for some time without suspecting it 
to be other than some torn fragment of seaweed. 
4 
There are many species belonging to several genera which fre- 
quent the Maine and Massachusetts coasts, but those most likely 
to be encountered are the following: 
Genus Dendronotus 
D. arborescens. This species is about one inch long and variable 
in color, but is usually reddish-brown or rose. There are no tentacles, 
but in their place are two antler-like appendages pointing forward and 
branched like a tree. All along the back are two rows of these curiously 
branched processes, which give to the animal the appearance of a plant. 
These cerata, or dorsal papille, are delicately transparent, contractile, 
and richly colored. The function of these papille is not fully known. 
As the animal has no specialized breathing-organs, it is reasonable to 
suppose that respiration is carried on through the outer skin and per- 
haps all over the surface of these branched papille. The liver, which 
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