Phylogeny of Animals 515 



or adult state is ciliate, the presence of four antennse two of 

 which in Nereis (as in some rotifers) have become palps, the 

 accessory peristomal setae, the ventral mouth and posterior 

 subdorsal or dorsal anus, a protrusible chitinous pharynx 

 which in rotifers bears the paired plates or mastax teeth that 

 seem to have become the jaws in Nereis, the accessory masti- 

 catory plates of rotifers that seem to correspond to the para- 

 gnaths of Nereis and other polychsets, an oesophagus with 

 paired sahvary glands, a brain in the prseoral region that gives 

 off nerve threads to the antennae and the eyes, the four eyes 

 that in some rotifers and in Nereis occupy similar positions 

 and are developed in fundamentally similar manner, an ex- 

 cretory system that in both shows hke structure to start with, 

 but which in adult polychsets like Nereis is greatly advanced 

 on, cement glands of rotifers that suggest exact correlation 

 with the parapodial glands of Nereis, and the formation in 

 Nereis, as in many rotifers at certain seasons, of gelatinous 

 investments from the cement glands inside which both ani- 

 mals survive. 



Fundamentally the Annelida have advanced beyond the 

 Rotifera, in increased development of the body tissues, in 

 formation of body segments and of added setse for these, in 

 the evolution of a blood-vascular system, and in increased com- 

 plexitj'^ of the excretory system. But the evolutionary con- 

 tinuity of members of both groups seems evident. 



Affinities of the Polyzoa with the Rotifera have been repeat- 

 edly suggested by zoologists, since the polyzoan larva passes 

 through a distinct trochophore stage. Some are soft-bodied, 

 and so would leave no fossil remains. These at the present 

 day are mainly fresh-water, others are calcified, and so have 

 left abundant remains from the base of the silurian rocks up- 

 ward. The latter are wholly marine. X few from both divi- 

 sions are solitary animals, but the great majority have, by 

 continuous budding, gradually evolved an extensive colonial 

 system. For the solitary state is naturally the more i)rimi- 

 tive, the colonial is more evolved. 



The changes which result, on fixation of the previously 

 free-swimming larva, as set forth diagrammatically in figs. 

 22, a-e, are in all cases so fundamental, and are further com- 

 plicated in most by extensive budding even in the larval state, 

 that the relation of the Polyzoa to the Rotifera has often been 



