OF MASSACHUSETTS. 41 



Anatomical Development. 



In order to insure a unified and connected narrative, it was thought 

 best, even at the risk of repetition, to trace the development of each 

 organ or set of organs separately. Wherever opportunity is given the 

 reader is referred to other portions of the report for supplementary 

 reading. In tracing the outline of the early life history of the scallop 

 the shell has been taken as the unit of description, and therefore its 

 development need not be treated separately, and only the soft or 

 internal parts of the animal need exemplification. Constant reference 

 is made to the various stages outlined in the table in chapter VII., and 

 to the illustrations, so as to present a connected account without unnec- 

 essary description. 



The Mantle. A description of the structure and functions of the 

 mantle of the adult scallop is given in chapter II., and it is only neces- 

 sary to recapitulate certain points which bear directly upon its develop- 

 ment. The mantle is a thin bilobed membrane closely lining the interior 

 of the shell and enfolding the body of the animal. The free edges form 

 thickened flaps, which are brilliantly colored and lined with rows of 

 sense organs, eyes and tentacles. The functions of the mantle are: 

 (1) shell secreting, as the growth of the shell is due to the secretions 

 from the mantle; (2) protective, as it enfolds and guards the body, and 

 is largely instrumental in swimming and feeding; (3) sensitory, as the 

 numerous tactile appendages and the circumpallial nerve render it 

 sensitive to the slightest stimulus. 



There is a steady development from the primitive mantle in the young 

 scallop to the highly specialized organ in the adult. It can be deduced, 

 from the changes which take place during the embryological and post- 

 embryological development, that the early ancestor of the scallop did 

 not have such highly specialized functions, which only developed when 

 the animal assumed its present dangerous mode of life, where it depends 

 upon its nervous mechanism to warn it of impending danger. 



The primitive mantle of the young scallop is a simple bilobed fold 

 joined along the hinge line, and is first visible in the prodissoconch or 

 late veliger stage. In the early veliger, although probably present to 

 enable the formation of the embryonic shell, it was not noticed. It 

 evidently attains prominence during the prodissoconch stage as a definite 

 mantle, common to all lamellibranchs, similar, except for the crenulated 

 edges, to that of the adult quahaug. At this time it appears entirely 

 separate from the degenerate velum, whereas in the early veliger it was 

 indistinguishable. The animal can extend the mantle slightly beyond 

 the shell, and by means of retractor muscles withdraw it to about two- 

 thirds its natural size. Even at this early period the mantle serves as a 

 sensory organ, as the edges are lined with minute cilia and simple 

 folds are already noticeable on the borders. 



