PHYLUM ANNELIDA 24I 



ment of complexity of structure. The appearance of this cavity 

 between the digestive tract and body-wall brought about great 

 physiological changes and is correlated with the origin of ne- 

 phridia for transporting waste products out of the body, and of 

 genital ducts for the exit of eggs and spermatozoa. The ccelom 

 also affected the distribution of nutritive substances within the 

 body, since it contains a fluid which takes up material absorbed 

 by the alimentary canal and carries it to the tissues. Excretory 

 matter finds its way into the ccelomic fluid and thence out of 

 the body through the nephridia. 



So important is the ccelom considered by most zoologists 

 that the Metazoa are frequently separated into two groups: (1) 

 the Accelomata without a ccelom, and (2) the Ccelomata with 

 a ccelom. The Porifera, Ccelenterata, and Ctenophora 

 are undoubtedly Acoelomata. Likewise the Annelida, 

 Echinodermata, Arthropoda, Molltjsca, and Chordata 

 are certainly Ccelomata. But whether the Platyhelminthes, 

 Nemathelminthes, and a number of other groups possess a 

 ccelom is still uncertain (see p. 25). 



The Trochophore. — The term trochophore has been applied 

 to the larval stages of a number of marine animals. The de- 

 scription and figures of the development of Polygordius (p. 233, 

 Fig. 162) are sufficient to indicate the peculiarities of this larva. 

 Many other marine annelids pass through a trochophore stage 

 during their life-history; those that do not are supposed to have 

 lost this step during the course of evolution. 



Since a trochophore also appears in the development of ani- 

 mals belonging to other phyla, for example, Molltjsca and 

 Bryozoa, and resembles very closely certain Rotifera, the con- 

 clusion has been reached by some embryologists that these 

 groups of animals are all descended from a common hypo- 

 thetical ancestor, the trochozoon. Strong arguments have 

 been advanced both for and against this theory. 



