306 



CCELHELMINTHES. 



worms with ccelom and with external and internal segmentation. 

 In the development there frequently occurs a type of larva, the 

 trochophore, which must be referred to here, since it is of great 

 morphological significance, resembling in structure the rotifers 

 and recalling the larva of the molluscs and to a less extent that of 

 the echinoderms. It is (fig. 273) a gelatinous body traversed by an 

 alimentary canal with fore-, mid-, and hind-gut regions. At first 

 it is everywhere ciliated, but with advance of development the 

 cilia become restricted to certain thickened tracts of epithelium, 

 the ciliated bands. One of these, the preoral band, is especially 

 constant. It runs circularly ( Wkr) around the body, surrounding 

 a circular prestomial area, in the centre of which is the anlage of 

 the cerebral ganglia, a thickened patch (apical plate) of ectoderm, 

 often bearing a tuft of cilia. Other ciliated bands (post-oral, 

 perianal) often occur. Of internal organs, besides numerous 

 muscle fibres, the most noticeable are the excretory organs, true 

 protonephridia, which open to the exterior either side below. The 

 trochophore in some respects resembles the larvae of some Turbel- 

 laria (fig. 231) and Nemertines (fig. 256), showing that the annelids 

 are related to these groups. 



The above account applies most closely to the Chsetopoda and 

 the closely related Archianellida. In other forms one or more 

 features may be lacking in the GephyraBa segmentation of the 

 body; in the Hirudinei most of the coslom and the trochophore. 

 Yet these are so closely related that they must be included under 



the common head; the missing 

 characters have been lost during 

 evolution. 



Sub Class I. Chcvtopoda. 



These, like the Nematoda, are 

 cylindrical worms, but are at once 

 distinguished by the segmenta- 

 tion. Deep circular constrictions 

 (fig. 274) bound the somites ex- 

 ternally. Internally the coelom 

 is divided by the septa delicate 

 double membranes which extend 

 from the ectoderm to the alimen- 

 tary canal into as many cham- 

 bers as there are metameres, while 

 a longitudinal mesentery, also double, separates the coelomic 



FIG. 274. Earthworm, side view and 

 anterior end enlarged. (After Vogt 

 and Jung.) 1, first segment with 

 mouth and prostomium; 15, male 

 sexual opening; 33-37, clitellum. 



