304 ZOOLOGY 



dilated vascular trunks, of which the heart is the narrowest, 

 and alone possesses the property of rhythmically con- 

 tracting." 



The heart is suspended in the pericardium by a number of 

 strands of tissue, and around these a curious collection of cells, 

 which has been compared to the " fat-body " of insects, is aggre- 

 gated. This tissue, from its distribution, has probably some 

 action on the blood, and it is possible that it represents 

 functionally the lymphatics of Vertebrates and the botryoidal 

 tissue of Leeches. 



The respiratory system is tracheal. Each trachea consists 

 of a simple opening to the exterior, — the stigma, — a short tube 

 which dilates at its inner end, and a number of very minute 

 tracheal tubules which arise from the dilated inner end of the 

 tube, and which are distributed to the tissues of the body. 

 The tracheal tubules are very fine, and for the most part un- 

 branched ; they exhibit slight traces of spiral striation. The 

 stigmata are more or less evenly scattered over the whole 

 surface of the animal, opening between the papillae ; there are, 

 however, a double dorsal and a double ventral row, and some 

 on the anterior and posterior face of each leg ; there is also a 

 large median ventral stigma just in front of the mouth ; this 

 supplies part of the nervous system. 



A nephridium occurs in the lateral section of the body- 

 cavity at the base of each foot (Fig. 175). Each nephridium 

 consists of the following parts : (i.) the external opening which 

 lies near the base of each leg ; this leads by a short duct into 

 (ii.) an expanded vesicle or collecting portion lined with large 

 flat cells ; from this a coiled tube (iii.) passes, which is lined 

 in various regions with four different kinds of epithelial 

 cells. This tube ends in a funnel-shaped internal opening, 

 the lips of which are continuous with the walls of a vesicle. 

 The study of the developement has shown that the lumen of 

 this vesicle, like the nephridium itself, is part of the original 

 coelom, which has become separated off from the rest (Fig. 

 176). The first two somites, those of the antennae and 

 claws, do not develope nephridia ; the nephridia of the third, 

 the oral papillae, are converted into the salivary glands ; the 

 next three segments have small, and the fourth and fifth pairs 



