TUBIFEX RIVULORUM. 17 



much searching to make their structure evident. The number of 

 segments varies much ; I have counted in some specimens from 

 60 to 80 of them. Each segment is separated from those adjoin- 

 ing by a muscular septum, closely constricting the intestine at 

 these points. It may be somewhat difficult for an uninitiated 

 observer to distinguish the head of the worm from its tail ; but 

 the position of the setae, directed backwards as the worm 

 advances, will show that the bluntly-pointed termination of the 

 body is the head, while the tail tapers away gradually and then 

 ends off abruptly truncated. 



The setae, with which every segment of the body, except the 

 head is furnished, are of two kinds — the long capillary setae, 

 which are confined to the dorsal surface ; and the hooked setae, 

 which are common to both surfaces. They are found in bundles, 

 of which each segment possesses one pair containing capillary, 

 and two pairs containing hooked setae. The latter are of the 

 forked shape, shown in Fig. 2. The number of setae in each 

 bundle varies somewhat, the capillary setae being usually two, 

 rarely three or four, — and the hooked varying from three to seven. 

 The hooked set^ are placed in pouches or invaginations of the 

 epidermis, to the bottom of which radiating muscular bands are 

 attached, by which their movements are effected (see Fig. 3). 



The integument is described by d'Udekem, as consisting of a 

 delicate epidermis, and of a chorion intimately united to the mus- 

 cular layer. The latter is divided into six longitudinal bands, 

 separated by as many furrows, as shown in the ideal transverse 

 section (Fig. 4). Upon these furrows are situated the bristle-sacs, 

 and into them the perivisceral cavity extends. This perivisceral 

 cavity occupies the whole of the large space which everywhere 

 intervenes between the muscular layer and the intestine. It is 

 lined. throughout with a cellular membrane, continuous, according 

 to Ray Lankester/'' with the glandular covering of the intestine, 

 of which we shall presently speak ; so that it may be described as 

 a double bag through which the intestine passes. On this account 

 it has been likened, by Dr. Williams,t to the peritoneal cavity of 

 the human body, and its contained fluid has received from 

 him the designation of the peritoneal fluid, or the chyl- 

 aqueous fluid of the peritoneal cavity. Between every two 

 segments it is greatly constricted by the muscular septum or 

 partition already referred to. The fluid with which this cavity is 

 fiUed is a highly coagulable and vital one. The coagulating prin- 

 ciple consists of fibrine, and the great bulk of the fluid portion is 

 composed of sea-water. Mechanically and physiologically, as Dr. 



* Observations on the Organization of Oligochaetous Annelids, Annals of 

 Natural Histoiy, 1871. 



t Report Brit. Assn., 1851. 



6 



