IV VERMES— BODY CAVITY 213 



ments of the Tiirhellaria, Nemeriina, and Hirudinea, divide the body 

 cavity into as many consecutive chambers as there are segments. 

 These transverse partition walls always run between 2 consecutive 

 segments. They are bored through by those organs which run 

 longitudinally through several segments ; viz. the enteric canal, the 

 blood-vessels, and the nephridia. It is the septa which bring 

 about the segmental constrictions of the intestine. The consecutive 

 chambers of the body cavity are seldom completely separated from 

 one another, the septa being mostly perforated, so that a free com- 

 munication of the coelomic fluid in adjacent chambers of the body 

 cavity is possible. 



All the walls of the body cavity and the organs lying in it are 

 lined with a peritoneal endothelium, which undergoes the most various 

 modifications. The endothelium of the body wall is distinguished as 

 the parietal layer ; that of the intestine as the visceral layer. 



The ohloragogen cells are peritoneal cells with definite excretory functions ; they 

 are especially strongly developed in the Oligochceta, and are attached to the dorsal 

 vessel and its branches, particularly to the network of blood-vessels which surround 

 the intestine. The brown gi'anules which they contain are products of excretion 

 taken from the blood, and most probably reach the exterior through the nephridia 

 by the detachment and dissolution of the chloragogen cells. We find such cells also 

 in the Polychceta. The excretory organs probably also draw the excretory substances 

 direct from the blood, i.e. from the network of vessels which surround. the renal 

 tube. 



The division of the body cavity may vary greatly in details. The dissepiments 

 may become reduced or wholly disappear in large tracts of the body, so that con- 

 secutive chambers of the body cavity coalesce. Especially where a protrusible pro- 

 boscis is developed, the segments through which this organ stretches undergo a 

 reduction of their dissepiments. Dissepiments and mesenteries are generally typic- 

 ally developed in an early stage, even where in the adult condition gi'eat transforma- 

 tions take place. 



The mesenteries may be reduced to isolated bands, fastening the intestine 

 to the body wall, and these may also he developed only in certain regions of the 

 body. On the other hand, the body cavity may undergo a still greater process of 

 division (especially in Polychceta). For example, a membrane running under the 

 intestine, attached on each side near the ventral chord to the body wall, often divides 

 the body cavity into an upper chamber containing the intestine, and a lower chamber 

 in which the ventral chord runs. Further, 2 lateral lamellfe often run through 

 the body in a dorso-ventral direction, slanting upwards and outwards from both 

 sides of the ventral middle line ; these cut off 2 lateral cavities from the body 

 cavity, which may be described as renal chambers, as they generally contain the 

 gi'cater part of the nephridia. These lamellfe enclose transverse muscle fibres (Fig. 

 158, p. 237). 



The dissepiments undergo a striking reduction, especially in the bodies of those 

 CJicetopoda in which the enteric canal forms loops, and in which the segmentation of 

 the body is more or less obscured (Olilorhcemidce, Stcrnaspidce, JEchiuridce). A spacious 

 body cavity is thus formed. In the Oapitellidce the want of a separate blood-vascular 

 system is compensated for by a strikingly pronounced partitioning of the bodj' 

 cavity. 



Communication between the body cavity and the outer world 



