414 THE ANATOMY OF IXVERTEBRATED ANIMALS. 



and larger blastomeres, of which the former, as an epiblast, 

 invest the latter as a hypoblast. At the cephalic end of the 

 embryo of most Lamellibranchs, a velum, or disk with richly 

 ciliated edges, and, usually, a central tuft of longer cilia, is 

 formed. On the dorsal face of the embryo the integument 

 rises into a patch with raised edges, which is the rudiment of 

 the mantle. The separation of the shell into two valves, 

 united by an uncalcified hinge, must probably be ascribed to 

 the manner in which the calcareous matter subsequently 

 added to the shell is deposited. The foot appears as a median 

 outgrowth of the neural face of the embryo behind the 

 mouth. The branchiae have, at first, the form of separate fila- 

 mentous processes, which are developed from the roof of the 

 anterior part of the pallial cavity, at the point of junction of 

 the mantle with the mesosoma, and gradually increase in 

 number from before backward. In those Lamellibranchs 

 w T hich have pouchlike gills, it appears that the processes 

 which are first formed become the outer lamella of the inner 

 gill-plate, their free ends uniting together; the inner lamella 

 of this plate is produced by the upgrowth of a thin lamina, 

 which subsequently becomes perforated, from the united ends 

 of these processes. The inner lamella of the outer gill is 

 formed of branchial processes, which grow out from the at- 

 tached ends of the first set ; and the outer lamella of this gill 

 is produced in the same fashion as the inner lamella of the 

 inner gill. 1 



Recent observations tend to show that in these, as in 

 other Invertebrata, the nervous ganglia are modified in- 

 growths of the epiblast. 



The simplest form of development of the Lamellibranchi- 

 ata has been observed in Pisidium? By the process of 

 cleavage, the vitellus is divided into a number of equal blas- 

 tomeres. The morula thus formed undergoes invagination, 

 and is converted into a gastrula. The blastopore, or aperture 

 of invagination, closes, and the epiblast, or ectodermal layer 

 of the embryo, growing much faster than the hypoblast, or en- 

 dodermal layer, the latter forms a small shut sac, the primi- 

 tive alimentary sac (or archenteron) attached to one point of 

 the inner surface of the much larger ectodermal sac. The 



1 Lacazc-Duthiers, " Sur le developpement des brnnchies des Mollusques 

 ac^phales Lamellibranches." (" Annales des Sciences Naturelles," 4, iv. ) 



2 Lankester, " On the Developmental History of the Mollusca." (" Phil. 

 Trans.," 1874.) 



