GASTEROPODS 355 
breathing-organs. In Doris, however, there are no dorsal papille at all, 
the back being covered with calcareous spicules, which form a sort of 
secondary shell. Thereis a cir- 
clet, or rosette, of retractile -  -. . 
plume-like gills, or branchim, %. ~ 4° 
placed in the middle of the back ~- : 
posteriorly. There are various * ' 
species of Doris, and of closely . 4% 
allied genera belonging to the &* 
family Doridide, which are tobe | : 
found along the New England Doris bilametlata. 
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 
