298 THE INVERTEBRATA 



often provided with lateral coeca, varying in number, and is used for 

 storing up the blood or other juices of the host. This is kept from 

 coagulating by the ferment (anticoagulin) contained in the salivary 

 secretion [Hirudo). In the mid gut a very slow digestion takes place, 

 the blood appearing almost unchanged even after several months. 

 (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 



rm 



Fig. 206, Glossiphonia as example of the Rhynchobdellidae. Dorsal view. 

 an. anus ; cr. crop (black) ; int. intestine (stippled) ; oe. oesophagus ; pb. pro- 

 boscis; ps. proboscis sheath; rh. rhynchodaeum ; rm. rectum; sa.gl. salivary 

 glands. 



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. Glossiphonia, Fig. 207 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 



