364 



ANNELIDA. 



the month a weaker (postoral) circle (Pow) ; to the right and left 

 there is an excretory canal (head kidney), which begins with a 

 ciliated funnel. By the differentiation of the cephalic region of the 

 larva into prsestomial lobe and oral segment, and by the gradual 

 growth in length of the posterior part of the body and the 

 -r, segmentation of the latter into a number 



of successive metameres, the originally un- 

 segmented larva is transformed into an 

 Annelid (fig. 294, ad). There is, therefore, 

 between the segmented adult and the larva 

 a morphological relation similar to that be- 

 tween the cestoid and the simple scolex, from 

 the posterior end of which the proglottides 

 are developed. 



The body of the Annelida is sometimes 

 flattened, sometimes completely rounded and 

 cylindrical. It is composed of a number of 

 successive segments, which are usually sepa- 

 rated from each other externally by trans- 

 verse constrictions. The segmentation is 

 generally homonomous^ in that the segments 

 following the head resemble each other not 

 only in external appearance, but also in 

 internal structure, i.e., they repeat similar 

 sections of the internal organization. The 

 terminal segment with the anus, however, 

 has a special structure inasmuch as it 

 retains the primitive, more indifferent char- 

 acter of the posterior end of the body of 

 the larva, and during the development of 

 the worm gives origin to new segments 

 anterior to itself. The homonomy of the 

 preceding segments of the body is, how- 

 ever, never complete, since certain organs 

 are confined to definite segments. The 

 internal segments, which are separated by 

 dissepiments, either correspond with the 

 external segmentation as marked by the 

 annular constrictions of the integument 

 (Chcetopoda), or each internal segment corresponds to a definite 

 number (3, 4, 5, etc.) of the external rings (Hirudinea). 



FIG. 294 d. The young 

 Polygordius ; G, cerebral 

 ganglion ; Wg, ciliated 

 pit ; D, alimentary canal. 



