METAZOA VERMES. 291 



is divided into a few distinct segments. Both the young 

 and the adult (fig. 2, D. gigas) are without tentacles and 

 no parapodia are developed. Neither are setae found, 

 and locomotion is effected by the bands of cilia clearly 

 seen in figs, i, 2. Practically, according to Harmer, 1 

 the adult is a trochophore, or, as Benham 2 puts it, "the 

 adult is more like a larval Polychaete than a full grown 

 worm." 



While it is true that there is a marked difference in 

 size between the male and the female of some species 

 (which is one reason why some naturalists consider 

 Dinophilus as a secondary rather than a primitive form), 

 nevertheless there are other species, like those figured in 

 PL 726, figs, i, 2. where, excepting the sexual organs 

 proper, the sexes are alike. 



Most Annelida arise from a trochophore which does 

 not differ essentially from that of Mollusca. As we have 

 already seen, this trochophore stage is common to sev- 

 eral groups of animals, Pelecypods, Gastropods, Cephal- 

 opods, Pteropods, Brachiopods, and for this reason we 

 maintain that it must be of phylogenetic importance. 



When the ancestors of the trochophore which were 

 active swimmers, gave up this mode of locomotion and 

 became crawlers, it is not unlikely that the body elongated. 

 Surely it is not difficult to conceive that such a change of 

 habitat and of habit would produce a long, more or less 

 flattened and unsegmented body. Neither is it hard to 

 see that, as time went on and the habit of creeping was 

 established, it would be an immense advantage to such a 

 crawling animal to have its body capable of the greatest 

 possible freedom of motion. This freedom might be 

 gained through purely mechanical means, whereby the 

 sinuous movements of the body would bring about a divi- 

 sion into parts or segments of greater and less mobility. 



1 Joum. Mar. Biol. Assoc., n. s., I, 1889, P- I 4 I - 



2 Cambridge Nat. Hist., II, 1896, p. 243. 



