EXCRETORY ORGANS OF VERMES. 



175 



ccelorn, or projects into it from the wall of tlie body, is composed of 

 two trunks (Fig. 81, c), which in many forms open by lateral branches 

 into the ccelom (species of Notommata). The two principal canals, 

 which are often much coiled, either unite at the cloaca, and open by 

 it to the exterior, or they pass into a contractile vesicle (r), which 

 must be regarded as a differentiation of the common terminal portion 

 of the two canals. The internal openings, as well as the lumen of 

 the two chief trunks, are here and there provided with nagelliforni 

 cilia, which produce a trembling movement. The walls themselves 

 may be seen to be glandular in character, either along the whole 

 length of the canal, or in certain portions only of it. In this last 

 point a further development of the simpler characters found in 

 the Platyhelminthes ought very probably to be recognised, which 

 points to a closer relationship with the Annelida. 



Echinoderes also possesses two coiled excretory tubes, but they 

 appear to open separately in the anterior region of the body. 



§ 144. 



In the Gephyrea we have to distinguish two kinds of excretory 

 organs. 



The first of these is an indication of the connection between the 

 Gephyrea, and lower stages, for it is 

 correlated with the undeveloped — or if 

 developed, developed only externally — 

 metamerism of the body. These organs 

 are formed by tubes which open into 

 the termination of the enteric canal 

 (Fig. 72, g), and are provided — at any 

 rate in the cases where they are most 

 exactly known (Bonellia) — with nu- 

 merous ciliated funnels, which open 

 into the ccelorn (Fig. 82, a). In other 

 cases the branches with internal open- 

 ings seem to be wanting (Echiurus), 

 and in others, again, the degeneration 

 is complete. As similar characters may 

 be seen in the Echinoderma, the form 

 of excretory organ, which obtains in 

 the Gephyrea, appears to be common 

 to a large circle, and referable to a 

 single typical form, whence it has been 

 transmitted to the Echinoderma, as well 

 as to the Gephyrea. Variety in the 

 function of this organ may be inferred 

 from its structure. The excretory 

 function is certain only in Bonellia, where the walls of its branches 

 have a glandular character. 



The other form consists of paired tubes, opening on the ventral 



Fig. 82. Portion of a branch of 



the excretory organ of Bonellia 



viridis. a Ciliated openings 



(after Lacaze-Dnthiers). 



