ON TWO NEW GENERA OF AQUATIC OLIGOCH^ETA. 279 



related to the fact that there are not a large number of such bodies in the former 

 genus. 



It appears to me also possible that these blood-glands are the physiological 

 equivalents of the "Herzkorper" of the Enchytrseidse and some Polyehseta. The cardiac 

 body in the former group is seen in two conditions — (1) a distinct, tubular, paired out- 

 growth from the alimentary tract lying on its dorsal side in Buchholzia appendiculata ; 

 (2) a solid rod in Mesenchytrceus extending through the greater portion of the dorsal 

 vessel. Michaelsen, who discovered the body in question in Mesenchytrceus [15], describes 

 and figures it as being attached to the ventral median line of the dorsal blood-vessel. He 

 makes no definite statement as to its continuity with the intestinal epithelium, but con- 

 siders that " it must be looked upon as an outgrowth of the intestinal epithelium into the 

 dorsal vessel, and, therefore, as homologous with certain organs in certain other Enchy- 

 trseidse, for example, the diverticulum of Buchholzia." In a later paper Miohaelsen [14] 

 noted the presence of a similar body in the dorsal vessel of Stercutus niveus. These dis- 

 coveries of Michaelsen are of great interest, as they confirm the suggestion of Horst 

 [23] that the cardiac body in certain Polyehseta is the homologue of the dorsal diverti- 

 culum of Buchholzia appendiculata. The structure of the cardiac body in Mesenchytrceus 

 is evidently much like that of Pectinaria belgica. Michaelsen at first [14] inclined to 

 the view that the solid cardiac body served the purpose of " purifying the blood from 

 useless, perhaps injurious, substances." This was also, as Michaelsen has pointed out, 

 the opinion of Claparede. 



A later suggestion of Michaelsen's, although highly ingenious, does not commend 

 itself to me as an improvement upon the earlier view. He says [14, p. 485]: — " Concerning 

 the meaning of the cardiac body I have lately formed an opinion which I will take this 

 opportunity of detailing. It is clear that undulatory contractions of a tube will drive 

 forward the fluid contained in that tube with a vigour proportionate to the narrowing 

 of the tube during each pulsation. If the lumen is fairly wide at the maximum of 

 contraction, a portion of the fluid contents will find a way out in the opposite 

 direction ; this backward flow will be completely hindered if the lumen is absolutely 

 obliterated during contraction. On the other hand, it is clear that long before this 

 point is reached the capability of contraction possessed by the tube will have found 

 its limits. To remove this difficulty dependent upon the limitation of contractility 

 and preventing a complete pulsation, a compact rod is formed in the tube. By this 

 means the walls of the tube, by closing round the rod, can obliterate the lumen of 

 the tube without reaching the limits of their power of contracting." 



This argument might, of course, be applied to the explanation of the lateral 

 cardiac bodies (or blood-glands, as I prefer to term them) of Phreodrilus ; but the 

 unequal distribution of granules, and the large size of the cells, and their apparently 

 vesicular nature, is against a purely mechanical interpretation of their function. 



Mr Cunningham has objected [22] to Horst's identification of the " cardiac body" in 

 the Chlorhsemidse with the gut diverticulum of the Enchytrseid, on the ground that there 



