2U COMPARATIVE ANATOMY chap. 



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

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

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

 clearly observed in the Lumbricicke and related land OUgocha'ta, and are 

 medio-dorsal perforations in the body wall, 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 Enchytrmidce ; and 

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

 the Oligochccta. But the presence of these pores has again recently been disputed. 



In the My-jostomidw a body cavity filled A^dth 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, Nemertinn, 

 and Turhellaria, a kind of muscle septa between which 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 intest^ine, with the uterus dorsally above it and the gan-, 

 glionio mass of the ventral chord under it. 



In the manner of division of the body cavity the Chcetognatha 

 are closely allied to the Choitopoda. 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 Pwsojyygia shows very different arrange- 

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

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

 cididce, is fastened to the body wall by delicate mesenterial strands 

 which are wanting in the Priapulidce. In Friupulns the body cavity 

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

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

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

 tine. The coelomic fluid generally contains amoeboid lymph cells ; 

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



