370 CH^ETOPODA. 



divisions or chambers, which correspond to the segments and dilate 

 again into lateral cliverticula and caeca. The constrictions are due to 

 filamentous or membranous septa (dissepimenta), which divide the 

 body cavity into the same number of chambers lying one behind 

 another. 



The vascular system appears to be closed, so that the clear nutri- 

 tive fluid found in the body cavity, which, like the blood, contains 

 amoeboid corpuscles, does not communicate with the usually coloured 

 contents of the vessels. The dorsal and ventral vessels are not only 

 connected at each end by lateral loops, but also in each segment ; and 

 from these connecting vessels proceed peripheral networks, which 

 extend into the integument, the wall of the alimentary canal, and the 

 branchiae. 



Special organs of respiration are wanting in almost all the Oligo- 

 chceta. In the marine Worms, on the contrary, branchiae are very 

 generally present, usually as appendages of the parapodia. These 

 branchiae are either simple cirri which have delicate ciliated walls 

 and contain blood-vessels, or are branched (Amphinome) or in some 

 cases are pectinate structures (Eunice) which co-exist with special cirri 

 on the notopodia (fig. 246). The branchiae are sometimes confined 

 to the middle segments (Arenicola), and are sometimes developed on 

 almost all the segments on the dorsal surface, being simplified 

 towards the posterior end of the body (Dorsibranchiata). In the 

 Tubicolce the branchiae are confined to the two (Peciinaria,Sabellidce) 

 or three (Terebella) anterior segments. The respiratory function is, 

 however, also shared (Ccqntibranchiata} by a number of elongated 

 tentacles wMch are grouped in tufts on the head. These are, in 

 the /Sabellidce, supported by a special cartilaginous skeleton, and 

 may have secondary twigs developed upon them. They are either 

 simply arranged in a circle round the mouth, or in two fan-like 

 lateral groups (Serpulidce), the base of which is not unfrequently 

 drawn out into a spiral plate. Such branchial structures, however, 

 also function as organs of touch, as organs for procuring nutriment, 

 and even for building the tubes and shells. 



Excretory organs. There are usually in all the segments paired 

 segmental organs, which serve as excretory organs. They begin, as a 

 rule, with a ciliated funnel in the body cavity ; they possess a glandu- 

 lar wall, are several times coiled upon themselves, and open to the 

 exterior in each segment by a lateral pore. These glandular 

 passages serve in general for the removal of matters from the 

 body cavity, and in the marine Chcetopoda are used during the 



