INTESTINAL RESPIEATION IN ANNELIDS. 771 



plexus, on both intestine and oesophagus, are the most obvious ; but the system com- 

 prises numerous and definite longitudinal and oblique vessels also. 



We thus have anatomical evidence that a very considerable proportion of the blood- 

 vessels in both lower and higher Oligochseta are derived from the alimentary plexus, 

 which, situated in the gut- wall, is the most primitive portion of the system ; the 

 vessels have arisen by a progressive differentiation of the channels in the plexus, 

 which at first, as still in y^ulosoma, consisted of a system of irregular lacunar spaces. 



Of the remaining portion of the vascular system, I propose to consider in detail 

 only the lateral loops or commissures, a certain number or all of which are, like the 

 dorsal vessel, contractile, and share in the function of propelling the blood. 



The Lateral Commissures. 



The direct evidence for the origin of the lateral loops from the alimentary network 

 is less than in the case of the vessels previously considered, and the earlier stages of 

 their development are not clearly documented for us. The argument must be largely 

 one of analogy. 



The contractility of the dorsal vessel is originally the same thing as the contractility 

 of the gut-wall ; and unless we are to assume an altogether different origin for the 

 contractility of the commissures, these also must be imagined as having their origin 

 in the gut-wall and as having later separated from it. 



Anatomically, it is perhaps worth while to recall such features as the parallel 

 channels running transversely between dorsal and subintestinal vessels in the crop of 

 CJiastogaster, and the well-marked transverse vessels in the intestine and oesophagus 

 of Phe7'etima and other earthworms. These examples show that not only longitudinal^ 

 but transverse channels also, may be formed in the gut-wall. From such channels 

 the commissures may be supposed to have arisen, separating themselves from the 

 alimentary wall in a manner like that of the dorsal and ventral vessels. As to why 

 such channels only separate from and become independent of the alimentary canal to 

 the number of one pair per segment, the answer is probably that this can only happen 

 where there is some support for them, along which they can travel, i.e. can only happen 

 at the positions of the septa. 



Is a Small or a La7'ge Numher of Commissures the Prim,itive Condition ? — In the 

 Tubificidge and in the higher families of the Oligochseta there is, speaking generally, 

 a pair of lateral loops to each segment of the body ; in ^olosom,a there are none, in 

 the Naididse and Enchytrseidse a few only, situated in the anterior region. Which 

 condition are we to look on as primitive ? In other words, are we to consider the small 

 number of loops in the Naididse and Enchytrseidse as the remnant of an originally 

 complete series, and is JEolosoma the last stage in the path of degeneration ? 



Let us consider first the case for degeneration. It has, in fact, been urged by 

 Beddard (3, pp. 72, 167, 170) that JEolosoma and the Naididse, though simple forms, 

 are secondarily simple, and not primitively so. 



