212 COMPARATIVE ANATOMY CHAP. 



wall. By the special arrangement of this partition wall (mesentery) 

 the body cavity is divided into 3 principal chambers ; 2 lateral, and 

 1 unpaired ventral chambers, in which the intestine runs. We shall 

 return to some peculiarities of this mesentery later on, when treating 

 of the genital organs. 



The presence of an endothelium and of a dorse-ventral median mesentery raises 

 the Gu-rdiida: almost to the level of the higher worms, and supports the view 

 that they should be considered as such (probably Annulata] degenerated by 

 parasitism. 



We cannot yet decide what should be considered as the body 

 cavity in the Himdinea among the Annulafti. The space between the 

 intestine and the body wall is filled by a connective tissue or paren- 

 chyma whose elements undergo the most varied transformations. 

 We find pigment cells, fat cells, fibres. Blood-vessels and blood-sinuses 

 arise by the flowing together of the contents of neighbouring cells. 

 The collective mass of the connective tissue elements is more strongly 

 developed in the Gnathobdettidce than in the Rhynchobdettidce. In the 

 latter division a connected system of blood-sinuses, whose walls are 

 not muscular but lined with an endothelium, and in which the 

 central nervous system lies, must be considered as a slightly developed 

 or else much reduced body cavity. In the Gfnathobdellidce the sinus, 

 which contains the ventral chord, the oesophageal commissures, and 

 the brain, seems to be such a reduced body cavity. This sinus, 

 however, is not lined with an endothelium. The fact that in 

 all Hirudinea the blood-vascular system is in open communication 

 with the sinuses makes it difficult to decide whether the canal 

 and sinus systems represent parts of the body cavity ; and if so, 

 to what extent this is the case. The sinuses are filled with a 

 fluid which, in the Rhynchobdellidce, contains colourless blood cor- 

 puscles. 



Muscle fibres, branched at both ends and attached to the dorsal 

 and ventral body walls, run through the body parenchyma. They 

 form muscular dissepiments between the enteric diverticula, the 

 arrangement of which recalls that of the dissepiments in the Nemer- 

 tina and Turbellaria, and, in correspondence with the metameric 

 arrangement of these diverticula, are themselves metameric. 



For the Chcetopoda we can establish a general morphological 

 scheme of the body cavity, which, however, undergoes considerable 

 modifications in a few divisions. Between the intestine and the 

 body wall there is always a body cavity filled with fluid, which is 

 entirely separated from the blood-vascular system. The body cavity 

 is divided in the following Avay. A dorsal mesentery connecting 

 the intestine Avith the dorsal middle line, and a ventral mesentery 

 connecting it Avith the A r entral middle line of the body Avail, divide 

 the body caA r ity into 2 lateral chambers, a right and a left. Muscular 

 partition walls, septa, or dissepiments, comparable to the dissepi- 



