H&MOCCEL AND C(ELOM u 



origin and essential nature, and the two systems have not even 

 secondarily acquired a connection with one another in either 

 Arthropoda or Mollusca. 



It is, therefore, very much to be desired that there should no 

 longer be any continuation of the confusion by the application of 

 the word "ccelorn" to the blood-sinuses of Arthropods or of 

 Mollusca. 



The independence of the origin of the " haemal system " or 

 "blood -vascular system" appears to be well established; but 

 it is by no means so clear as to what is the history of the first 

 beginnings and subsequent development of the haemal system in the 

 animal series, as might be supposed. Whilst we are able to form 

 some conception of the probable history of the vicissitudes of the 

 coelom from its first appearance to its present condition in the 

 various phyla of Coelomocoela, we find that few, if any, attempts 

 have been made to trace out the history of the haemal system 

 in the same series. It is probable that it is one and the same 

 morphological entity, which we recognise as the blood-vascular 

 system or haemal system, in Vertebrata, Mollusca, Arthropoda, 

 Chaetopoda, Nemertina, and Echinoderma. Its function is essen- 

 tially the absorption and distribution of chemical substances im- 

 portant in the life of the tissues, among the first of these being 

 oxygen gas. How could such a system originate 1 As ramifying 

 capillary channels or as simple longitudinal trunks ? It is certain 

 that the walls of simple blood-vessels, and the blood itself, are 

 closely related in nature to the connective tissues, and in some 

 cases they have been shown to be developed from such tissue. 

 Possibly the earliest vascular system was preceded by solid rami- 

 fying cords of connective tissue, which performed absorptive and 

 distributive chemical functions even though not yet tubularised 

 and differentiated into liquid content and enclosing wall. We 

 have no conclusive reason for supposing that the haemal system 

 must have taken origin within the grade of Ccelomocoela. It is 

 quite possible that we have to look for its origin in the lower 

 grade of Enterozoa the Enterocoela. This is a subject upon 

 which much speculation is possible, but to which little serious 

 attention has as yet been given. That the haemal system is 

 connected in origin with a space which often arises between the 

 two primitive cell-layers of the embryo (the blastocoel) has been 

 suggested on the ground of certain embryological observations, 

 but the embryological facts are not in themselves conclusive as 

 to the ancestral arrangements of the parts in question. This 

 question is further considered below under the section " Crelom and 

 Mesenchvme." 



