THE GASTROPODA 



133 



types such as Paludina, Planorbis, etc., the endoderm formed by the 

 macromeres is invaginated into the ectodermic layer formed by the 

 micromeres, but in many cases, in consequence of the far more rapid 

 multiplication of the micromeres and the much larger size of the 

 macromeres, gastrulation is effected by epiboly, and the endoderm is 

 invaginated at a later period; in this case the endodermic cavity 

 or enteron is of small size. In most cases the blastopore closes, 

 and the definitive mouth is formed by a new invagination at the 

 point of closure . it is only in Paludina that a portion of the blasto- 

 pore remains open and becomes the anus (Fig. 110, C, bl ; F, an). 

 The mesoderm is formed as two primary mesomeres from the more 

 posterior of the two primitive macromeres (Fig. 11, mes). The 

 mesodermic organs (definitive kidney, heart, etc.) do not arise until 

 a late period, their place being taken, during the development, 

 by provisional larval organs, 

 such as superficial contractile 

 sinuses and larval kidneys 

 (Figs. 114, 118). 



The ciliated trochosphere 

 larva is sometimes consti- 

 tuted at a very early period, 

 before the formation of the 

 mesoderm e.g. in Patella, 

 Acmaea, and Trochus and in 

 such cases the embryo be- 

 comes free at once. But in 

 most cases the larva is not mc ~ 



ot 



Kn 



Pio. 111. 



Young veliger of Troclius, ven- 

 tral aspect. /, foot ; m, mouth ; 

 ]M, mantle ; pa.c, pallial cavity ; 

 *h, shell ; ve, velum. (After 

 Robert.) 



FIG. 112. 



Larva of Cavolinia triilentata, ventral aspect. 

 a, anus; /, median portion of the foot; h, 

 heart ; i, intestine ; K u, contractile sinus ; 

 TO, mouth ; mb, mantle-skirt ; me, subpallial 

 chamber; ot, otocyst; pn, lateral lobe of the 

 foot (the future left fin) ; q, shell ; r, kidney ; 

 s, oesophagus; <r, sac containing nutritive 

 yolk. (From Lankester, after Fol.) 



hatched out until a much later period, and a large part of the 

 development is effected within the egg envelope. The larva whose 

 shell is often different from that of the adult is characterised by 

 its velum and by some other external or superficial larval organs. 



