2 52 POLYCHAETA 



vessels pass to the intestinal wall and to the body-wall, and here 

 form extensive capillary networks (Fig. 124, p. 247). This type 

 of vascular system is pretty generally adhered to throughout the 

 Order, but in the Terebelliformia, Scoleciformia, and Crypto- 

 cephala the dorsal vessel and capillary plexus on the intestine 

 are replaced by a continuous blood sinus, situated in the sub- 

 stance of the gut-wall. This " perienteric sinus " has the same 

 relation to the segmental vessels as the dorsal vessel has in the 

 Nereidiformia> and from it a tubular dorsal vessel arises ante- 

 riorly. In Arenicola the sinus is preceded in the young stage 

 by a network the branches of which gradually enlarge, meet, and 

 fuse to form the sinus. 1 Whether it is in all cases secondary is 

 a moot point. 



This system of vessels in the majority of Chaetopoda contains 

 a respiratory fluid coloured red 2 by haemoglobin in solution ; in it 

 float a very few small oval nucleated non-amoeboid corpuscles. But 

 the place of this red pigment is taken by a green one, named 

 " chlorocruorin," in the Chlorhaemidae and many Sabelliformia ; 8 

 whilst in Magelona 4 the blood is tinted madder-pink by a number 

 of globules of " haemerythrin." The blood (or " haemal fluid ") is 

 driven forwards in the dorsal vessel, and passes backwards in the 

 ventral vessel. Eespiration in Nereis is carried on by the whole 

 surface of the body, but naturally with greater activity in the 

 surface of the parapodia, the lobes of which, with their extensive 

 vascular plexus, may be termed " gills " ; but it must be borne in 

 mind that these 'organs have other functions as well. 



The coelomic fluid, which fills the general body-cavity, is 

 colourless, and contains amoeboid corpuscles or " leucocytes." It 

 corresponds to the lymph of Vertebrates, being nutritive in func- 

 tion, in that it conveys absorbed material from the wall of the 

 intestine to the organs of the body, and at the same time removes 

 any waste substances from these organs ; these waste substances 

 contain nitrogen, and are ultimately removed by the nephridia. 

 In Ophelia many of the corpuscles contain a curious dumb-bell- 

 shaped rod of chitin, and it has been shown 5 that this substance 



1 Benham, "The Post-Larval Stage of Arenicola ," J. Mar. Biol. Assoc, iii. (n.s.) 

 1893, p. 48. 



2 The blood is colourless in Syllidae and Nephthydidae. 



3 Ehlers states that some Eunicidae have green blood. 



4 Benham, Quart. J. Micr. Sci. xxxix. 1896, p. 1. 



5 Schaeppi, Jena. Zeit. xxviii. 1894, p. 217. 



