246 THE LAMELLIBRANCHIA 
This condition is reached during the veliger stage; the shell, 
which is at first too small to contain the whole animal, is called 
the prodissoconch, and is characterised by its two symmetrical 
valves with a simple linear hinge. After this stage a sudden 
Fie. 223. 
Development of Ostraea edulis. A, blastula stage, with commencing invagination at bl, the 
blastopore. B, optical section of a somewhat later stage, in which the invagination of the shell- 
gland, sk, has commenced; bl, blastopore; ec, ectoderm; en, endoderm. C, similar optical 
section of a little later stage. The invagination connected with the blastopore is now more 
contracted, @; and cells, me, forming the mesoderm, are separated. D, similar section of a 
latter stage (trochosphere) with closed blastopore bl; m, the mouth; s, shell, on the surface of 
the shell-gland sk. H, surface view of an embryo ata period a little more advanced than D. 
F, the same embryo seen as a transparent object. a, anus; e, intestine ; ft, foot; m, mouth ; 
sk, shell-gland ; st, stomach ; tp, velarareaof the prostomium. The extent of the shell and com- 
mencing upgrowth of the mantle-skirt is indicated by a line forming a curve from ato F. On 
the dorsal side of the stomach is the anterior adductor muscle. (From Lankester, after Horst.) 
change is effected in the secretory activity of the mantle, and 
the embryonic shell or prodissoconch is often separated from the 
rest of the shell by a more or less projecting ridge, indicating 
this modification during growth (Fig. 196, p). (2) The velum, which 
serves as the larval swimming organ, is a circular outgrowth with 
