CHAETOPODA 



157 



Fig. 100, 



Hedone 

 Sav. 

 After Savigny. 



" siphon," that is, a tube which opens at both ends into the 

 alimentary canal. The siphon never contains food, and its 

 function is probably respiratory. A similar 

 structure runs from one part of the alimen- 

 tary canal to another in the Echiuridae, 

 and in Echinoids. The Capitellidae and 

 some other families are without any blood 

 system. In other worms the principal vessels 

 are similar to those described in Arenicola ; 

 the red blood of some forms is due to 

 haemoglobin dissolved in the plasma, in 

 others the blood is green or almost colourless. 



The typical arrangement of the nephridia, 

 one pair in each segment, is often iaterfered 

 with. They usually fail in the anterior 

 segments when there is a large pharynx, and 

 in the tubicolous forms their number is 

 usually much reduced : e.g. eight pairs in 

 Terebella and six pairs in Arenicola. The 

 genus Capitella are remarkable for having 

 several pairs of nephridia in each segment, the number increas- 

 ing in the posterior end up to six or seven pairs in this genus. 

 The nephridia are themselves subject to much variation ; 

 one nephridium may have several funnels, and may be con- 

 nected by a tube with another, and sometimes the organ 

 breaks up into small tubules. The whole arrangement recalls 

 the excretory system of some of the earthworms described 

 above. 



The ventral nerve cord of some of the tubicolous Poly- 

 chaeta has its right and left half divaricated, and connected 

 by numerous transverse commissures. This is well shown in 

 Serpula, and in a less degree in Sabella. Eyes are very generally 

 present, and are usually confined to the prostomium. Foly- 

 ophthalmus, however, has a number of lateral eyes, a pair to 

 each somite ; whilst Branchiomma bears them on its branchial 

 filaments. Otocysts, such as those of Arenicola, are rare. 



The Polychaets, with some exceptions, are dioecious. The 

 generative organs are usually developed in relation to a blood- 

 vessel, which no doubt serves to nourish them ; in the Seden- 



