764 PROFESSOR J. STEPHENSON ON 



independence of the alimentary canal ; the intestinal network becomes a less prominent 

 portion of the whole system, and lateral loops, parietal vessels, and branchial 

 vessels are developed. The physiological independence of the two systems also 

 increases hand in hand with the anatomical ; in the lower stages there is one 

 series of postero-anterior contractions common to the two systems, while in the 

 higher a separate series of vascular contractions is becoming differentiated from the 

 alimentary series. 



The Tubificidae present us with further stages in this differentiation of the two 

 systems. The anatomical summaries given under the headings of the three species 

 here described show two things : first, that the degree of complexity attained by 

 the vascular system is considerably greater than in such a typical Naid as Nais ; 

 secondly, that Clitellio, in having a continuous ventral vessel throughout the body, 

 in possessing lateral loops which do not branch, and in being without either a cutaneous 

 plexus or a system of branchial vessels, is at a lower level than the other species. The 

 physiological observations correspond ; they show that in Clitellio a very distinct 

 connection between alimentary and vascular contractions is still to be observed — the 

 degree of independence of the two being, in fact, less than in the Naid Branchiodrilus ; 

 but they show also that in the species with a still more developed vascular system, the 

 alimentary and vascular contractions are now at length entirely independent of each 

 other. The common contractility of the intestinal wall and included vascular elements 

 has become differentiated into two parts — that of the alimentary wall, and that of the 

 dorsal vessel. The character of the contractility possessed by these two structures 

 is not the same : that of the dorsal vessel is the more marked, being more energetic, 

 more frequent, and more rapid ; while the contractility of the intestine seems to 

 be losing in importance, since it is frequently absent, and when present, is more 

 limited in extent than in the iEolosomatidae, Enchytraeidae, and Naididse. 



Higher Oligochxta. 



A full description of the vascular system in the earthworms need not be given ; 

 as is well known, and as may be illustrated by the description given by Fuchs (19) 

 for Lumbricus, a still higher degree of development has been attained, and a still 

 higher degree of independence of the alimentary canal. A reminiscence of an earlier 

 anatomical condition is however still retained, in the close connection of the dorsal 

 vessel with the ijitestinal wall; thus Fuchs : "Das Gefass (Vas dorsale) verliiuft 

 mediodorsalwarts liber dem Darm, diesem eng angeschmiegt, mit iVusnahme der 

 Pericordalregion. . . . Sowohl das Vas dorsale auf ganzer Lange besonders seitlich 

 als diese Dorsointestinalia sind mit Chloragogen bedeckt." 



The species which is studied as a type of Oligochseta in the colleges of Northern 

 India, and also at Calcutta, is Pheretima postJiuma ; and this being the form with 

 which I am most familiar, it will be convenient here to describe shortly the vessels 



