MOLLUSKS 151 



free-swimming veliger larvie, 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 jDOwer 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. 



Grepidula plana is a small, flat, white species which lives 

 within shells that are bping 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 eonvexa 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 little 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 Grepidula is given by E. Gr. Conklin in the 

 "American Journal of Morphology," Vol. XIII, Part 1, 1897. 



THE NAKED MOLLUSKS OR SEA SLUGS. 



Niidlbranchiata. 



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 larvae are cast oil very 

 early in development. 



The mouth is on tlie lower side nf the front end of the body, 

 and the intestine usually gives off branched canals which ramify 

 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 wliich are often 

 highly colored and resemble a beautiful flower. These gills are, 



