GASTEROPODS 355 



breathing-organs. In Doris, however, there are no dorsal papillae at all, 

 the back being covered with calcareous spicules, which form a sort of 

 secondary shell. There is a cir- 

 clet, or rosette, of retractile ^ r- 

 plume-like gills, or branchiae, 

 placed in the middle of the back 

 posteriorly. There are various 

 species of Doris, and of closely 

 allied genera belonging to the 

 family Dorididfp, which are to be 

 found along the New England 

 coast in tide-pools and among 



rocks. Those seen by the writer never appeared to be other than very 

 sluggish creatures, and proved to be rather unsatisfactory tenants of the 

 aquarium. 



When discovered at low tide upon moist seaweed or stones, or 

 when disturbed in the water, nudibranchs often appear like small 

 lumps of jelly-like tissue, without a single attractive feature. It 

 is only when they are placed in a jar of sea-water and left un- 

 molested for a few moments that they unfold their beauties to the 

 view. On the Pacific coast there are numerous species of nudi- 

 branchs, occupying corresponding shore stations and exhibiting 

 the same degree of high coloration. There are over a thousand 

 species of sea-slugs scattered about the various seas. They find 

 their greatest development in tropical waters, and for brilliancy 

 of tint and variation in form are unequaled by any other class of 

 animal life, save perhaps the butterflies. 



ORDER PROSOBRANCHIATA 



To this exceedingly large and comprehensive order of gastero- 

 pods belong the great majority of univalve mollusks. In point 

 of number of genera and species and in abundance of individuals, 

 the prosobranchs probably exceed all the other orders of mollusks 

 combined. They may be called the typical gasteropods, the 

 shell being univalvular and generally spiral, and the animal ex- 

 hibiting the singular torsion of the visceral mass which has placed 

 the breathing-organs forward of the heart. They are provided 

 with an operculum (placed upon the posterior dorsal portion of 

 the foot), which is used to close the aperture of the shell against 



