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, 01; 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 
ve pgay 
‘ ee Gs 
CTT 
[f= 
oh Fic. 112. 
fierce 2 
Larva of Cavolinia tridentata, ventral aspect. 
Fia. 111. a@, anus; f, median portion of the foot; h, 
heart; 7, intestine; Kn, contractile sinus; 
Young veliger of Trochus, ven- m, mouth; mb, mantle-skirt ; me, subpallial 
tral aspect. f, foot; m, mouth ; chamber ; ot, otocyst; pn, lateral lobe of the 
pa, mantle; pa.c, pallial cavity ; foot (the future left fin); g, shell; 7, kidney; 
sh, shell; ve, velum. (After 8s, oesophagus; o, sac containing nutritive 
Robert.) 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. 
