246 



COMPARATIVE ANATOMY 



CHAP. 



ill / 



Annulata. This correspondence indeed is apparent in the whole 



nephridium. 



The only organs which can perhaps be pointed to as transformed 

 nephridia in the Chcetognatha are the ovi- 

 ducts and the sperm ducts; these are 

 paired tubes, the oviducts opening out- 

 wardly, at the posterior end of the trunk 

 segment, the sperm ducts in the tail 

 segment by paired lateral apertures. 

 The latter are provided with funnel-like 

 ciliated inner apertures. 



X. Respiratory Organs. 



In many worms no specific respiratory 

 organs are developed. Respiration is 

 performed by means of the integument, 

 and also often by the walls of the intestine. 

 The general ciliation of the body in the 

 Nemertina is of great assistance for respira- 

 tion in water. In the Hirudinea and 

 many Oligochce-ta cutaneous respiration is 

 facilitated by the presence in the integu- 

 ment of numerous fine blood-vessels. But 

 we can only speak of specific respiratory 

 organs where definite organs have been 

 developed whose exclusive or most im- 

 portant function is respiration. Among 

 the Hirudinea we see in Pontobdella the 

 integumental capillaries of the blood - 

 FIG. 162. organisation of Dino- vascular system already localised in eleva- 



md 



pharynx ; 'phd, pharyngeai glands ; n, therefore be called the branchial papillae. 

 nephridia; md, mid-gut; wk, ciliated j n Bmnchellwn each ring carries on each 



rings ; o. ovarium ; an, anus ; ed, hind- . -, , -IT > , i ' t 



gu1 r side a branched appendage into which 



blood-vessels enter. Specific respiratory 



organs are wanting in the Oligochceta and the Archiannelida. Among 

 the Oligochceta, only the insufficiently known Alma nilotica possesses 

 branchial appendages at the posterior part of the body. The Polychceta, 

 on the contrary, are pretty generally supplied with gills, which 

 are usually branched appendages of the Parapodia (Fig. 124, p. 188 ; 

 Fig. 158, p. 237); these in some cases, however, may emancipate 

 themselves from the parapodia and may be independently inserted 

 on the back. The comparative morphology of the gills of the Polychceta 

 has not yet been thoroughly worked out. We may perhaps at once 

 distinguish two sorts of gills lymph gills and blood gills. The 

 lymph gills are processes of the parapodia which are distinct from 



