PHYLUM ANNELIDA. SEGMENTED WORMS 



193 



Coelenterata, Ctenophora, and Platyhelmin- 

 thes are undoubtedly Acoelomata. Likewise 

 the Annelida, Echinodermata, Arthropoda, 

 Mollusca, and Chordata are certainly Coelo- 

 mata. The Nemathelminthes and related 

 phyla belong to the Pseudocoelomata. 



Trochophore 



The term trochophore has been applied 

 to the larval stages of a number of marine 

 animals. The figures of the trochophores of 

 the polychaete Eupomatus (Fig. 107) and 

 of Polygordius (Fig. 103) are sufficient to 

 indicate the characteristics of this larva. 



Many other marine annelids pass through 

 a trochophore stage during their life his- 

 tory; 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 animals belonging to other 

 phyla, for example, Mollusca and Br}^ozoa, 

 and resembles very closely certain Rotifera, 

 the conclusion has been reached by some 

 embr)'ologists that these groups of animals 

 are all descended from a common hypo- 

 thetical ancestor called a trochozoan. Strong 

 arguments have been advanced both for and 

 against this theory. 



Apical organ 



Esophagus 



Mesenchyme 



Stomach 

 Ciliated finQ 



Larval nephn'dium 

 Otocyst 



Blastocoel 

 Ciliated ring 



Anos 

 Anal vesicle 



Figure 107. Trochophore larva of a polychaete, Eupomatus, side view. 



ORIGIN AND RELATIONS 

 OF THE ANNELIDA 



The annelids comprise the polychaetes, 

 archiannelids, oligochaetes, and leeches. 

 Formerly the archiannelids were regarded as 

 ancestral annelids. It was hypothesized that 

 both polychaetes and oligochaetes evolved 

 from them. Since the discovery that the 

 archiannelids show some larval features that 

 resemble larval polychaetes, it is not known 

 whether they are primitive or degenerative. 

 Hartman prefers to regard the Archiannclida 

 as an Appendix of Polychaeta. 



The polychaetes are by far the oldest, 

 largest, and most diversified of the annelids. 

 The origin of aquatic and terrestrial oligo- 

 chaetes from an ancestral, generalized 

 polychaete is likely. The leeches, in turn, 

 have many features in common with oligo- 

 chaetes; their peculiar modifications are the 

 result of parasitism. 



RELATIONS OF THE 

 ANNELIDA TO MAN 



Of the influence of segmented worms on 

 human welfare, that of the earthworm and 



