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 Pacific coasts of North America, and more especially in the 

 colder waters north of Cape Cod. They are essentially littoral, 

 and live upon algas 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 algae, 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. 



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 papillae, are delicately transparent, contractile, 

 and richly colored. The function of these papillae 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 papillae. The liver, which 



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