MOLLUSKS 151 
free-swimming veliger larvee, but after two or three weeks they 
settle down as young boat-shells. Until they are about half grown 
they can move slowly, but finally they become fixed for the remain- 
der of their lives. ‘The males are smaller and more active than the 
females, but they also lose all power of locomotion when more than 
half grown. This species may be recognized by its decidedly con- 
vex shell, gray, horny color, and faint reddish-brown flecks over 
the surface of the shell. 
Crepidula plana is a small, flat, white species which lives 
within shells that are being carried about by hermit crabs. The 
head end of the crepidula almost always faces the opening of the 
shell within which it lives. Its breeding habits are similar to 
those of C. fornicata. 
Crepidula convexa is the smallest of our species, and its shells 
are dark brown in color, and more convex than either of the others. 
It lives on the outside of shells, being carried about by hermit 
crabs. It can move to a limited extent throughout life, and the 
young hatch as httle snails, and do not pass through a free-swim- 
ming veliger stage. 
A most thorough account of the development and life history 
of our species of Crepidula is given by E. G. Conklin in the 
“American Journal of Morphology,” Vol. XIIT, Part 1, 1897. 
THE NAKED MOLLUSKS OR SEA SLUGS. 
Nudibranchiata. 
These are slug-like in general appearance but are not to be con- 
fused with land slugs. They have a flat creeping foot, and no shell 
when adult, although one is present in the young, indicating that 
they have descended from sea snails. However, the shell and 
operculum which are found in the young larve are cast off very 
early in development. 
The mouth is on the lower side of the front end of the body, 
and the intestine usually gives off branched canals which ramitfy 
through the body. The vent is either upon the middle line of the 
back or upon the right side. When found upon the mid-dorsal 
line it is usually surrounded by branching gills which are often 
highly colored and resemble a beautiful flower. These gills are, 
