398 



HIRUDINEA 



fact that there are considerable remains of the body-cavity or coeloni 

 which form a complicated system of spaces and channels. What 

 has happened, in fact, in the leech is that the coelom has become 

 gradually and partially obliterated by proliferation of the cells in 

 the interior of the body, a process of obliteration which has already 

 commenced in the Oligochaeta. In many of the latter, some of the 

 principal blood-vessels have become surrounded by a space cut off 

 from the general body-cavity, while in the majority a special 

 cavity surrounds the testes and the funnels of the sperm-ducts. 

 This process of the formation of separate cavities for the inclusion 

 of the several viscera culminates in the leeches with the marked 

 obliteration of the greater part of the coelom. This has become 

 so much reduced to the condition of narrow tubes that there has 

 been a tendency to confuse it with the vascular system, more 

 especially perhaps in those forms in which the blood is tinged 

 with haemoglobin, and in which there is a connexion between 

 the two systems of spaces. This confusion has been further 

 increased by the plan of injecting the vascular system, a method 

 of investigation which must be employed with great care in 

 delicately-organised creatures whose tissues can be easily ruptured, 

 and so lead to a flow of the injecting fluid into places and in 

 directions impossible during life. 



In transverse sections of leeches it may be seen in successful pre- 

 parations that the various organs of the body are enclosed in spaces. 

 The funnels of the nephridia open into lacunae which could hardly 



in any case be regarded as blood 

 spaces, while the blood-vessels 

 themselves with their mus- 

 cular walls cannot be con- 

 founded, at least in the case 

 of the larger trunks, with the 

 spaces not having muscular 

 walls which surround them. 

 Furthermore, it will be pointed 



FIG. 206. Coelomic canals of Glossiphonia out immediately that the 

 cowplanata. x 10. (After Oka.) a, Dorsal , ,. 



canal containing dorsal blood-vessel; 6, reproductive organs are pro- 



ventral canal containing ventral blood- duced On the Walls of Spaces 

 vessel : I, lateral canal : n. nerve-cord. , . 



which are the commencement 



in the embryo of the reduced coelom of the adult worm. These 

 spaces therefore conform in every particular to the general con- 



