CHIRONOMUS PRASINUS. 169 



The Vascular System and Ccelom. — It will be well, 



perhaps, shortly to compare these parts of the organism as they 

 are found in the Vermes and Arthropoda respectively, observing 

 that a reference to my paper on Tub if ex riindoriiin "■'" will help to 

 illustrate many of the points mentioned. The ccelom of the 

 Vermes is marked off into sections almost separated by the 

 inter-segmental septa, whilst that of the Arthropoda is continuous. 

 The vascular system of the former is closed and quite cut off 

 from the coelom ; that of the latter ij-c.^ the dorsal vessel) is in 

 communication with it. The vascular fluid of the former differs 

 in colour, etc., from that occupying the coelom ; that of the latter 

 is identical with it. The dorsal vessel of the former is in close 

 connection with the intestine ; that of the latter is separate 

 therefrom. The coelom of the former is circumscribed by an 

 endothelial lining, the peritoneal membrane of Dr. Williams ; 

 that of the latter is not so circumscribed, but extends between the 

 muscles and other organs. These differences show that it must 

 not be concluded, as might happen from a superficial view, that 

 the ccelomic cavity of the Arthropoda is homologous with that of 

 the worms, and Gegenbaur states that the coelom of the former is 

 part of the vascular system. f 



The dorsal vessel, therefore, of our larva may be regarded as 

 part and parcel of the vascular body-cavity in which it lies. It 

 consists of a chambered tube commencing in the twelfth, or 

 penultimate segment of the larva, and extends forwards towards 

 the head ; its anterior termination I have not been able to trace. 

 It is curious that while in many allied larv?e this tube appears to 

 be contractile throughout its length, the contractiHty in this case 

 is, I think, confined to the first chamber (reckoning these in a 

 backward direction) in the twelfth segment, where it is very 

 marked, and is evidently due to the presence of a coating of 

 transverse muscles which produces the systole. How the diastole 

 is brought about is not equally obvious, but I think it is by the 

 elasticity of a number of almost invisible connective tissue cords 

 that connect the dorsal wall of the chamber with the integument. 



* vSee this Journal, Vol. L, p. 14. 

 t Gegenbaur's Elements of Comparative Anatomy, p. 27S, English translation. 



