TYPE ANNELIDA. 219 



rich in gland-cells and also containing numerous sensory 

 cells connected at their inner ends with slender fibrils which 

 pass centrally and unite with others to form nerve-cords pass- 

 ing to the ventral nerve-cord. In some of the segments near 

 the anterior portion of the body the hypodermis is usually 

 thicker than elsewhere and richer in gland-cells, forming a 

 structure known as the clitellum (Fig. 103, c). Within the hypo- 

 dermis is a usually thick layer of circular muscles, and, internal 

 to this, longitudinal muscles whose continuity as a layer is in- 

 terrupted at the dorsal and ventral mid-lines as well as at the 

 sides. Muscular dissepiments divide the ccelom into compart- 

 ments corresponding to the external segmentation of the 

 body, and the various compartments are lined by a layer of 

 peritoneal cells which along the dorsal and ventral lines is 

 reflected towards the digestive tract, which it surrounds, form- 

 ing a dorsal and ventral mesentery. The former of these fre- 

 quently disappears, as may also the ventral one. The dissepi- 

 ments are rarely perfect, being usually perforated so that the 

 various ccelomic compartments are placed in communication 

 with each other, and in rare cases a number of the dissepi- 

 ments may be wanting, as in jEolosoma, where but a single 

 one separating the head from the trunk ccelomic spaces oc- 

 curs. On the median dorsal line of more or fewer metameres 

 towards their posterior edge a small opening usually occurs, 

 the dorsal pore, and similar pores are found in the head 

 segment of some forms. They place the ccelomic cavitj in 

 communication with the exterior, but do not seem under ordi- 

 nary circumstances to be the means of any extensive inter- 

 change between the external water and the ccelomic luerno- 

 lymph, though occasionally this latter fluid may find exit 

 through them. Among the peritoneal cells are usually to be 

 found some which enclose greenish-brown particles and may 

 detach themselves from the peritoneal layer and float about 

 in the hsemolymph, eventually dying and disintegrating. Oc- 

 casionally these chloragogm cells are specially aggregated in a 

 furrow which runs along the dorsal surface of the intestine, 

 and immediately surround the dorsal blood-vessel. They 

 seem to be excretory in function, performing perhaps to a cer- 

 tain extent the part of the liver-cells of the Vertebrata. 



