196 THE ANATOMY OF INVERTEBRATED ANIMALS. 



(except in the most anterior part of the body), this connective 

 tissue is continued transversely toward the axis of the bodv, 

 and passes into that which forms the wall of the intestine ; 

 while, on the ventral side, it forms an arch over the ventral 

 nervous cord and the vessels which accompany it. In the 

 interior of each of these mesenteric septa, radiating and circu- 

 lar muscular fibres are abundantly developed, and the former 

 are connected externally with the superficial layer of trans- 

 verse muscles. 



The perivisceral cavity is thus divided into nearly as many 

 short chambers as there are segments; each chamber com- 

 municates with the exterior, directly by the dorsal pore and 

 indirectly through the segmental organs, while fluid may pass 

 from one to the other by the supra-neural archways. 



The short and curved setae project much farther into the 

 interior of the body than they do on to its exterior. The free 

 apices of each pair are situated close together, while their 

 inner ends diverge from one another. Each is inclosed in a 

 sac in which it is developed, and to which the muscles, by 

 which it is protruded, are attached. There are eight setae to 

 each somite, one pair not far from the ventral median line on 

 each side ; and the other pair placed in the same transverse 

 line, but further outward. 



The mouth leads into a muscular pharynx, with a com- 

 paratively small internal cavity, which reaches as far back 

 as the seventh segment. From this a narrow oesophagus is 

 continued as far back as the fifteenth or sixteenth segment ; 

 and presents three pairs of lateral glandular diverticula, which 

 contain a calcareous matter, 1 in the region of the twelfth and 

 thirteenth segments. Posteriorly, the gullet opens into a 

 crop, which is succeeded, about the eighteenth segment, by a 

 thickened and muscular gizzard. 



Upon this follows the intestine, which has the appearance 

 of a simple tube ; but is in reality complicated by the invo- 

 lution of its wall, along the dorsal median line, into a thick 

 fold, which projects into the interior of the intestinal cavity, 

 and is the so-called typhlosole. The exterior of the intestine 

 and the cavity of the typhlosole present a coating of yellow- 

 ish-brown cells. 



The segmental organs are greatly convoluted tubes, situ- 



1 The nature of this substance has recently been discussed by M. E. Perrier, 

 " Etude sur un genre nouveau des Lombriciens." ("Archives de Zoologie ex- 

 pe'rhnentale," 1873.) 



