PHYLOGENY 257 



choanoflagellates. Proterospongia (Fig. 50 B) is a unique form 

 which might resemble the ancestors of the Porifera and in an 

 unusual manner stands intermediate between the choanoflagel- 

 lates and the sponges. This is a colony of choanoflagellates the 

 individual cells of which are embedded in a matrix containing 

 some wandering amoeboid cells comparable to the mesoderm cells 

 of the Porifera. 



Coelenterata and Ctenophora. — It seems probable that a 

 primitive gastrula of the nature of Haeckel's hypothetical 

 Gastraea must have had its origin from a spherical protozoan 

 colony by more rapid growth at one pole which resulted in an 

 invagination to form the gastrula cavity. The planula, some 

 modification of which occurs in the larval development of all 

 coelenterates and of the ctenophores, is but a slightly modified 

 gastrula. As MacBride has so well pointed out, a planula-like 

 ancestor probably gave origin to the members of these two phyla. 



Plathelminthes.^ — ^In an earlier chapter it has been stated 

 that Coeloplana (Fig. 59) and Ctenoplana are modern genera 

 which stand intermediate between the flatworms and the cteno- 

 phores and display an odd combination of characters some of 

 which had come to be considered as diagnostic for one or the 

 other of the two phyla. It is probable that through some form 

 similar to these the flatworms have arisen from a ctenophore-like 

 stock. 



Nemathelminthes. — Regarding the origin and relationships of 

 the Nemathelminthes practically nothing is known. They 

 stand as peculiarly isolated forms whose history has never been 

 deciphered. The method of mesoderm formation seems to 

 preclude any close relationships with the annehd worms. There 

 has been no well-founded theory advanced as to their origin or 

 relationships. The numerous suggestions which have been 

 offered are based upon very minor details. 



The Trochophore. — In the Plathelminthes, Coelhelminthes, 

 Molluscoidea, Trochelminthes, and Mollusca there occur larvae 

 which represent only minor modifications of the trochophore 

 (Fig. 76). Thus a direct relationship among these forms is 

 traceable through the larvae. In fundamental structure the 

 trochophore has been likened to the ctenophores. It has already 

 been shown that the Plathelminthes are readily derivable from 

 the ctenophore organization. The metamerism, so characteristic 

 of the Annelida, has been explained on various grounds. Begin- 



