214 COMPARATIVE ANATOMY CHAP. 



takes place in two ways. Firstly, by the nephridial canals, which 

 will be described later, and which are originally present in pairs in 

 each segment, and, secondly, by the dorsal pores. These have been 

 clearly observed in the Lumbricidiv and related land Oligochceta, and are 

 medio-dorsal perforations in the body Avail, lying in the anterior end 

 of each segment. They are wanting in the head segment and in a 

 certain number of the subsequent anterior segments. 



According to some writers, dorsal pores are present also in JSnchytrceidce ; and 

 cephalic pores leading into the head cavity are to be found in different families of 

 the < il iijiK-liii fn. But the presence of these pores has again recently been disputed. 



In the Myzostomidcs a body cavity filled with fluid is wanting. 

 The organs found between the intestine and the body wall, above 

 all the genital organs, are embedded in a body-parenchyma of con- 

 nective tissue. Still the question remains to be decided, whether the 

 space in which the sexual products lie does not answer to the body 

 cavity of other worms. Dorso-ventral muscle fibres run through the 

 parenchyma and form, in the same way as in the Hirudinea, Nemertina, 

 and Turbellaria, a kind of muscle septa between Avhich secondary 

 muscle septa coming from the edge are intercalated, in the spaces 

 between the sexual organs and the enteric diverticula. In the middle 

 region of the body the septa leave a considerable space open in which 

 we find the intestine, with the uterus dorsally above it and the gan- 

 glionic mass of the ventral chord under it. 



In the manner of division of the body cavity the Chcetognatha 

 are closely allied to the Chcetopoda. The body cavity is divided by 

 '2 dissepiments into 3 consecutive chambers. The first dissepiment 

 lies at the boundary between head and trunk, the second between 

 trunk and tail, and the 3 chambers separated by the septa are the 

 head, trunk, and caudal cavities. The enteric canal divides the head 

 and trunk cavities into 2 lateral portions ; in the trunk cavity the 

 intestine is often fastened to the body wall by a dorsal and a ventral 

 mesentery ; a dorso-ventral mesentery-like partition of the caudal 

 cavity is also found, although the intestine is there wanting. The 

 parietal and visceral layers of the peritoneal endothelium are continued 

 on to the mesenteries and the dissepiments, and form their chief com- 

 ponent part, as they are not provided with muscles. 



The body cavity of the Prosopygia shows very different arrange- 

 ments. That of the Sipunmlacea is large and spacious, like that of 

 the Echiuridce; dissepiments are wanting. The intestine, in the Sipnu- 

 culida'., is fastened to the body wall by delicate mesenterial strands 

 which are Avanting in the PriapuUdtr. In Priapuhis the body cavity 

 is continued into the caudal appendage. A large expanse of the 

 peritoneal covering of the intestine is ciliated in the Sipunculidce, and, 

 as in the Echiuridce, a longitudinal muscular band runs along the intes- 

 tine. The ccelomic fluid generally contains amoeboid lymph cells ; 

 and besides these, in the Sipnnculidce, the sexual products and other 



