116 INVERTEBKATA CHAP. 



Of course there are differences. Thus there are eight, not four, 

 nmcromeres, in Ctenophora, and according to the best account we 

 have of their development the ectoderm is separated into two sets 

 of micromeres, not three sets. 



But Surface has brought out clearly a hitherto unsuspected 

 agreement between the two groups, viz. the origin of most of the 

 so-called mesoderm. This material is really similar in both groups, 

 consisting of stellate cells with processes, some of which are con- 

 tractile. In both groups it results principally from cells budded from 

 the macromeres, after the ectodermal material has been separated off; 

 and the small cells budded from the lower poles of the macromeres of 

 Ctenophora, after they have been almost covered by ectoderm, may 

 well be compared to the fourth quartette of the Polyclade. 



If we then accept provisionally this theory of Lang's, we are led 

 to an interesting conclusion. We saw (p. 51) reason to assume the 

 existence of a free-swimming, blastula-like, ancestral form, of which the 

 Porifera were concluded to be degenerate sessile descendants. In the 

 same way, the development of a gastrula from the blastula was shown 

 to be evidence that the whole group of the Coelenterata are descend- 

 ants of this pelagic group. Of this pelagic group, when it had attained 

 the Coelenterate stage of development, the greater portion, as we saw in 

 the last chapter, took to a sessile life, but one portion, the ancestors 

 of modern Ctenophora, retained their -free-swimming life. We now 

 conclude that from this group of what we may term primitive 

 Ctenophora, a set of degenerate descendants arose which gave rise to 

 the Platyhelminthes. 



The history of the Metazoa, so far as we have yet traced it, is that 

 of a main pelagic group increasing in complexity of structure as time 

 goes on, and at each level throwing off creeping and sessile stocks 

 which are more or less degenerate in structure. We have indicated 

 that the Polyclada are the only group of Platyhelminthes from the 

 study of whose development much light can be expected on the origin 

 of the phylum ; but our knowledge is extremely defective even of the 

 development of the Polyclada. 



The student here meets with a phenomenon which will recur 

 throughout the course of his study of Invertebrate Embryology, and 

 it is this, that while the early development of the egg up to the 

 period when the embryo becomes free, is known with some accuracy, 

 next to nothing is known of the larval and post-larval stages. 

 The reason for this ignorance lies chiefly in the difficulty found in 

 obtaining appropriate food for the larva when the embryonic stores 

 of nourishment are exhausted. This difficulty has been overcome in 

 the case of Echinodermata, and further study will, no doubt, enable 

 us to overcome it for the other groups also. 



As a consequence of this state of affairs, however, there are vast 

 gaps in our knowledge of the development of every group of Inverte- 

 brates ; these gaps can and should be filled up. They offer a most 

 promising field for further work. Thus, for example, in the develop- 



