12 CCELOM AND BLOOD- VESSELS 



(c) Intercommunication of Codom and Blood-vascular System. ' 



To return to the ccelom. Whilst there is no direct communi- 

 cation between that cavity and the haemal system in Arthropoda 

 or Mollusca, yet such a communication does occur in two import- 

 ant* groups of Ccelomocoela. In the Vertebrata the lymphatic 

 vessels are in more or less direct communication with the ccelomic 

 cavity, and also open into the haemal system at more than one 

 point. The condition in Amphioxus, as described by Schneider, is 

 such as to give a very free communication between the vascular 

 system and the coelomic space at the base of the hepatic caecum. 

 It would be desirable that the existence of this connection in 

 Amphioxus should be inquired into again, though there seems to 

 be little doubt as to its existence. 



Among the Chaetopoda two very striking facts as to the fusion 

 of coelom and haemal system have been recognised. The first is 

 the breaking up of the haemal tissue in Glycera and the Capitellidae 

 in such a way as to result in the total disappearance of the haemal 

 system as a series of vessels whilst its cell -elements remain as 

 corpuscles coloured red by haemoglobin and floating in the coelomic 

 fluid. The second is the assumption in certain of the Leeches of 

 a canalicular form by a large part of the coelom and the junction 

 of the canals so formed with the true haemal system by means of 

 capillaries. A remarkable fact is that portions of the coelom (the 

 perigonadial portions) are shut off from this combination. We 

 thus obtain in the Leeches in question a uniform fluid, impregnated 

 in most cases with haemoglobin, circulating in vessels some of 

 which are of haemal and others of coelomic origin. The fact that 

 such a free intercommunication exists has been both asserted and 

 denied, but the most recent careful investigations (Goodrich, Quart. 

 Journ. Micr. Sci. 1899, vol. xlii. p. 477) leave no doubt that it really 

 obtains. So long as it was held that coelom and haemal system were 

 one in origin, and that a fusion of the two obtained in Mollusca 

 and Arthropoda, the case of the Leeches did not appear singular. 

 But our present conception as to the complete independence of the 

 two systems in origin, and the knowledge that they do not inter- 

 communicate in either Mollusca or Arthropoda, renders it desirable 

 that we should have, if possible, a greater certainty than we have 

 at present as to the developmental origin of the channels which 

 are ascribed to coelom in such Leeches as Hirudo. The evidence 

 appears to be in favour of their coelomic origin, but it is just 

 possible that they are not coelomic. In Acanthobdella and also in 

 Clepsine (the former of which is to be regarded as an archaic form) 

 the haemal system is entirely closed and coexists with a well- 

 developed coelom into which it does not open. 



