METAZOA 145 



The yolk, which varies in amount in all phyla, is responsible for 

 much of the difference between animals at corresponding stages, 

 especially the youngest. The student will recall that from this cause 

 the cleavage of the ovum which in Amphioxus is complete and equal, 

 is in the frog complete but unequal and in the chick incomplete. 

 The blastula has in Amphioxus a large cavity, in the frog a small 

 one, in the crayfish (Fig. 202) is full of yolk, and in the chick is a 

 disc upon the yolk. The mode in which the establishment of the 

 two-layer stage (gastrulation) takes place is also partly affected by 

 yolk, though evidently other factors are concerned. In the Planula 

 (Fig. 152) it is by immigration, in Amphioxus and the crayfish by 

 invagination, in the frog largely by overgrowth (epiboly), in the chick 

 by delamination. 



All these modes of development are repeated sporadically in 

 various groups of Metazoa: thus the early stages of the mollusc 

 Paludina and the crustacean Leucifer are analogous to those of 

 Amphioxus^ those of the squid and the scorpion, members of the 

 same phyla, to those of the chick. On the other hand, certain features 

 of cleavage are constant through whole phyla and groups of phyla. 

 The cleavage of coelenterates and echinoderms is radial (Fig. 196, i), 

 that of chordates is bilateral, that of polyclads, nemerteans, annelids, 

 and molluscs is spiral (p. 281). Determinate cleavage, in which the 

 part of the body to be formed by each blastomere is fixed from the 

 first, as in the case described on p. 282, is common to the spirally 

 cleaving groups but occurs in a quite different manner in the tunicates. 

 Again, while the mesotheHum of annelids, arthropods, and molluscs 

 is laid down as a pair of ventral bands proliferated from behind, 

 that of other coelomata arises from the wall of the definitive enteron 

 (p. 129). 



An important function of many larvae is the distribution of the 

 species. This is often effected by their being planktonic. Among the 

 larval types adapted to that existence is a series whose members have 

 delicate tissues and a large blastocoele, whereby their buoyancy is 

 increased, and strongly ciliated bands, often drawn out into processes, 

 whereby swimming and feeding take place. To this series belong 

 Miiller's larva (Fig. 155), the Pilidium (Fig. 169), the trochospheres 

 (Figs. 197, 374, 420), the Actinotrocha (Fig. 432), the Dipleurulae 

 (Fig. 439), and the Tornaria (Fig. 466). The student should beware 

 of supposing that these types are phylogenetically related. With one 

 or two exceptions, their resemblance is probably an instance of 

 convergent adaptation. 



