2 4 6 



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 



FlO. 223. 



Development of 0$tr<iea tdvlis. 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, d\ and cells, me, forming the mesoderm, are separated. 1), similar section of a 

 latter stage (trochoaphere) with closed blastopore bl ; m, the mouth ; s, shell, on the surface of 

 the shell-gland sk. E, surface view of an embryo at a period a little more advanced than D. 

 F, the same embryo seen as a transparent object, a, anus ; e, intestine ; ft, foot ; m, mouth ; 

 tk, shell-gland ; st, stomach ; tp, velar area of the prostomium. The extent of the shell and com- 

 mencing upgrowth of the mantle-skirt is indicated by a line forming a curve from a to 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,^). (2) The velum, which 

 serves as the larval swimming organ, is a circular outgrowth with 



