12 CCELOM AND BLOOD- VESSELS 



(c) Intercommunication of Coslom and Blood-vascular System. 



To return to the coelom. AVhilst there is no direct communi- 

 cation between that cavity and the haemal system in Arthropoda 

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

 ant groups of Ccelomoccela. 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 cojlomic 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 ccelom and hremal system have been recognised. The first is 

 the breaking up of the htemal tissue in Glycera and the Capitellid;v 

 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 ccelomic 

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

 a canalicular form by a large part of the crelom 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 ccelom (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 ccelomic 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 hremal 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 tlir 

 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 ca4om in such Leeches as Hirudo. The e^^dence 

 appears to be in favour of their co?lomic origin, but it is just 

 ])ossible that they are not ccelomic. In Acanthobdella and also in 

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

 the hi\!mal system is entirely closed and coexists with a well; 

 developed ccelom into which it does not open. 



