22 CONK LIN. [Vol. XIII. 



The cleavages are precisely the same in all the species up to 

 the 52-cell stage. At this point the ectoderm cells begin to 

 grow more numerous in adunca, though the divisions continue 

 the same until a still later period in the other species. 



In all the species the number of mesoderm and entoderm 

 cells remains the same as far as they can be recognized. 



There can be no doubt that C. plana and C. fornicata, with 

 their larval types of development, represent a more ancestral 

 condition than C. convexa and C. adunca with their suppressed 

 larval or foetal type.^ It must be considered that the larval 

 type of development is the more ancestral, from which the 

 foetal type has been derived. The small number of eggs and 

 the direct development of C. convexa and C. adunca are corre- 

 lated with the small size of the adult in these species, and this 

 in turn may be due to the action of environment through 

 natural selection. These species live upon small objects, 

 chiefly those small gasteropod shells like Litorina or Chloros- 

 toma, which are inhabited by the small hermit crab, and only 

 those individuals could survive in these positions which are 

 small enough to become firmly attached to these shells, while 

 all larger ones would be torn off, and would sooner or later 

 perish. The dwarf variety of C. plana furnishes evidence that 

 the cause here assigned for the small size of C. convexa and 

 C. adunca is not purely imaginary. The ability which all the 

 members of this genus show to adapt themselves to large or 

 small places, and to modify the shell so that it will fit plane, 

 convex, concave or angular surfaces indicates, that the body is 

 very plastic. 



But whatever the cause of the smaller size of C. convexa 

 and C. adunca may be, it is evident that the total mass of 

 germinal matter must be less in these than in the larger 

 species, provided that all the other organs are developed in 

 about the same relative proportions, as appears to be the case. 



^The use of the expression "foetal type of development" in this case is, I 

 think, justifiable. It is true that in these four species various stages in the sup- 

 pression of the larval development are shown, and even in that species in which 

 the larval development is most completely suppressed, viz., C. adunca, there are 

 many rudiments of larval organs ; yet these are only rudiments and they com- 

 pletely disappear before the young escape from the capsules. 



