VASCULAR SYSTEM BODY-CAVITY 



397 



tion to the number of segments out of which this portion of the 

 body is possibly formed. It will be ob- 

 served also that the dorsal vessel is double 

 in this region, a condition which obtains 

 along its whole length in Branchellion a 

 repetition of what has been described in 

 more than one species of Oligochaete. In 

 the region of the last pair of digestive 

 caeca the dorsal vessel has appended to it 

 copious sinuses which embrace the intestine 

 and supply its walls with blood. In Hirudo 

 there are only a pair of lateral vessels, and 

 neither dorsal nor ventral vessels ; in this 

 leech and in the Gnathobdellae generally 

 there are intra-epidermic capillaries, a fact 

 first discovered by Professor Lankester, and 

 now known to occur also in the Oligochaeta. 



The development of the blood-vessels 

 shows that they have no relation whatever to 

 the coelom, in spite of their subsequent con- 

 nexion with it. The two longitudinal stems of 

 Hirudo arise as cavities in the somatic layer 

 of the mesoblast after the formation of the 

 coelom. In Nephelis, but not in Hirudo, Dr. 

 Burger thinks that there is some reason for 

 regarding the vascular system as the remains 

 of the primitive segmentation-cavity of the 

 embryo, an opinion which is held in respect of 

 the vascular system of many other animals. 



Body-Cavity. One of the most marked differences between the 

 leeches and the Oligochaeta is in the body-cavity. In the latter 

 there are a series of cavities corresponding to the segments, which 

 are bounded in front and behind by the intersegmental septa, and in 

 which all the viscera lie. In leeches such an arrangement is not 

 recognisable save in Acanthobdella, where Kowalevsky 1 has quite 

 recently described a typical coelom divided by septa into twenty 

 segments. In transverse sections the body of other leeches appears 

 at first sight to be solid, owing to the growth of the muscles and 

 connective tissue. A more careful study, however, has revealed the 



1 See ref. on p. 395. 



Fig. 205. Glossiphonia 

 margi?iata, vascular and 

 alimentary system, x 4. 

 (After Oka.) a, Dorsal 

 vessel ; g, intestinal 

 caecum ; v, one of the 

 hearts. 



