CHAETOPODA 265 



(4) An intestine, which is also endodermal, and has, in Hirudo, a pair 

 of diverticula. (5) A very short ectodermal rectum discharging by the 

 anus, which is dorsal to the posterior sucker. 



The body wall consists of a single layer of ectodermal cells between 

 which blood capillaries penetrate, a dermis with pigment cells and 

 blood vessels, and an outer circular and inner longitudinal layer of 

 muscles. The muscle fibres have a characteristic structure, consisting 

 of a cortex of striated contractile substance and a medulla of un- 

 modified protoplasm. Inside the musculature are masses of mesen- 

 chymatous tissue : in the Gnathobdellidae this is pigmented and forms 

 the botryoidal tissue, the cells of which are arranged end to end and 

 contain intracellular capillaries filled with a red fluid. 



The mesenchyme almost completely occupies the space which is 

 the perivisceral cavity in the earthworm. There are, however, longi- 

 tudinal canals, constituting the sinus system, and these represent the 

 remnants of the coelomic spaces ; there are always dorsal and ventral 

 and often (e.g. in Clepsine, Fig. 196 B) two lateral sinuses, and there 

 are numerous transverse canals in each segment. Into this reduced 

 coelom the nephrostomes open and the gonads are found in it. The 

 blood system consists of two contractile lateral vessels (and in the 

 Rhynchobdellidae of dorsal and ventral vessels running inside the 

 corresponding coelomic spaces). These vessels all communicate with 

 one another. They also communicate with the sinuses of the coelom 

 and with the capillaries of the botryoidal tissue, as has been shown 

 by careful injection. This astonishing condition is unique, but a 

 parallel may be drawn with the vertebrate in which the lymphatic 

 system communicates both with the coelom and the blood system. 

 The peculiar functions of the lymphatic system are not shared by 

 the botryoidal vessels which have no particular connection with the 

 gut. 



The nervous system is of the usual annelidan type but characterized 

 by the fusion of ganglia anteriorly (Fig. 194) and posteriorly. There 

 are segmental sense organs in the form of papillae, and on the head 

 some of these are modified to form eyes and the so-called ''cup- 

 shaped organs". 



The nephridia consist of two tubes, one ending in a nephrostome, 

 the other with an external aperture ; their lumina do not communicate 

 (Fig. 181); the nephrostomes open into a branch of the ventral or 

 the lateral sinus. The testes, of which there are often several pairs 

 (nine in Hirudo), and the single pair of ovaries are also present as closed 

 vesicles in the sinuses and are derived from the coelomic epithelium, 

 but in distinction from the rest of the annelids they are continuous 

 with their ducts. The separation of the genital part of the coelom from 

 the rest, begun in the Oligochaeta, here becomes complete. The 



