POLYCHAETA. 465 



(Terebellids, Cryptocephala, etc.). The blood is usually red, owing 

 to the presence of haemoglobin in solution ; but in some forms 

 (Cldorliaemidae, and some Sabettiformia) it is green, owing to the 

 presence of a green pigment called cldorocruorin. In Aphrodite 

 the blood fluid is yellow, and in Magelona it is pink, owing to 

 the presence of a pigment called liaemerythrin. The vascular system 

 is said to be absent in the Glyceridae, Capitellidae, Polijcirrus, and 

 Tomopteris. 



The respiratory function is discharged by the gills (see above), 

 and probably by the whole surface of the skin. 



The perivisceral cavity (coelom) is well developed, and is 

 frequently divided by transverse segmental septa into chambers, 

 but in some of the tubicolous forms the septa are deficient, many of 

 them being absent. Muscular bands pass from the median ventral 

 line, where they are inserted on each side of the nerve cord obliquely 

 dorsally and outwards, where they spread out in a fan-like manner to 

 be inserted into the dorso-lateral body-wall. They divide the body- 

 cavity more or less completely into three regions, two lateral, and a 

 median containing the alimentary canal (Fig. 364). The coelomic 

 fluid contains amoeboid corpuscles, and is usually colourless; but in 

 the Glyceridae, Capitellidae, Pott/cirrus, the coelomic corpuscles are 

 coloured with haemoglobin. 



Median dorsal and ventral mesenteries passing from the body-wall 

 to the gut-wall, and dividing the body-cavity into a right and left 

 half, are present in some forms (Capitellidae, Sabella, etc.). 



Excretory organs. In many Polychaets the nephridia occur in all 

 the segments except a few of the anterior and posterior ; but in some 

 they are reduced in number, e.g., to six pairs in Arenicola, one pair 

 in Sternasjris, and one or two pairs in Clilorliaemidae. In the 

 ( 'u/iitellidae there may be more than one pair in a segment, and a 

 single nephridium may have more than one internal opening ; more- 

 over, adjacent nephridia may be connected together. 



The nephridia of Polychaets are either long convoluted tubes 

 (e.g., Nereis, Fig. 370) with a small internal funnel, or short and 

 wide tubes (e.g., Arenicola, Fig. 371) with a wide funnel. The 

 nephridia of the former kind cannot serve as escape-ducts for the 

 ova, and it becomes a question, in animals Avhicli possess them alone, 

 how the eggs pass outwards.* In some tube-dwellers one or more 



It is possible that the dorsal ciliated organs of Nereis, which occur in pairs 

 in most of the segments as ciliated patches of coelomic epithelium, may really 

 have undetected external apertures (Goodrich, Q.J.M.S., 34, 1893.) 



2 H 



