164 INVEETEBEATA CHAP. 



Polygordius, which is deducible from the anatomy of the adult, is fully 

 borne out by a study of its development. 



When we turn to the development of the Annelida for light as to 

 the origin of the phylum, the question to be solved resolves itself 

 into the question of how the Trochophore larva is to be interpreted. 

 We have already anticipated the answer to this question in dealing 

 with the Pilidium larva of Nemertinea ; we interpreted that larva as 

 representing, in a simplified form, an ancestor which, if it were living 

 now, would be classed as a Ctenophore. 



But the Trochophore of .Polygordius, at the stage when mouth and 

 anus are still not separated, presents an even closer resemblance to a 

 primitive type of Ctenophore than does the Pilidium larva. For the 

 four discrete groups of ciliated cells which have not as yet united to 

 form the prototroch, may be regarded as representing shortened 

 ciliated ribs. Further, Woltereck has shown that beneath the ciliated- 

 apical plate are ganglion cells and fibres, and that from this centre, 

 radial nerve strands extend out to the groups of ciliated cells which are 

 supposed to represent the ribs, just as nerve strands radiate from the 

 polar plate underlying the ribs in Ctenophora. 



The likeness of the larval muscles to the contractile strands of 

 Ctenophora has already been dwelt on in the case of the Pilidium, 

 and need not be further insisted on here, but the fact that four, not 

 eight ribs are developed in the Trochophore need not disturb us, 

 especially in view of the fact (see p. 92) that the eight ribs of the 

 Ctenophore are represented in the embryo by four radiating streaks 

 of small cells, and that it is therefore independently probable that in 

 some ancestral Ctenophore there would be four, not eight ribs. 



We should thus have three distinct larval types, viz. Muller's 

 larva, the Pilidium, and the Trochophore, all representing, with more 

 or less modification, a group of ancestral Ctenophora from which 

 sprang the Polyclada (and through them all Platyhelminthes), the 

 Nemertinea and the Annelida. 



The Polyclada are essentially ground Ctenophora which glide over 

 the substratum instead of swimming freely through the water, and 

 Ctenoplana seems to be a modern Ctenophore beginning to undergo 

 a change similar to that undergone by ancestral Ctenophores when 

 they became Polyclada. 



As to the nature of the changes which the ancestral Ctenophore 

 underwent in becoming a Nernertinean, we shall not hazard an 

 opinion until the metamorphosis of those worms has been more 

 thoroughly analyzed. 



Woltereck, however, attempts to sketch the course of the changes 

 which the Trochophore underwent in becoming an Annelid. He 

 supposes that as it grew older the animal developed the habit of 

 dropping to the bottom and taking to a burrowing habit, whilst still 

 retaining its free-swimming habits during its youth. As a conse- 

 quence the trochophoral ciliated cells were rubbed off, and the gap in 

 the ectoderm thus caused was closed before and behind by prolifera- 



