INTESTINAL RESPIRATION IN ANNELIDS. 825 



inheritance from swimmers ; in tlie same strain, referring to Amphioxus, he says that 

 however primitive one may hold its organisation to be, yet nobody will assert that its 

 limbless body and its predominantly cryptoid locomotion represent archaic features. 



WiLLEY, however, disagrees with Eisic. : " I am unable," he says, "to follow these 

 conclusions myself, as I approach this matter independently from a cryptozoic and 

 stereotropic standpoint"; Willey would thus presumably derive the free-swimming 

 habit of, say, Phyllodocids from crawling locomotion, of which it would merely be an 

 exaggerated development. Finally, he reaches "the conclusion that just as all divisions 

 of the animal kingdom display a cryptozoic bias, so do they all show a statozoic 

 tendency" (Statozoa = sedentary animals). "The sedentary habit is referable to a 

 stereotropic basis, and pelagic or pleotropic forms belonging to groups which are 

 predominantly sedentary, have had a. probable statozoic origin." 



If the above line of reasoning is correct, the absence of parapodia, i.e. of organs of 

 active locomotion, in the sedentary forms will not be, as is perhaps generally assumed, 

 due to degeneration ; it will be a primitive feature. In other words, the sedentary 

 Polychseta have never had parapodia. And, in fact, well-developed parapodia occur 

 in only a relatively small minority of the families of Polychaeta ; and in these again 

 it is only a portion which have the double-branched parapodium, consisting of 

 notopodium and neuropodium, which, in the form in which it occurs in Nereis, 

 constitutes the type of the organ which is presented to the student. Parapodia {I 

 refer, of course, to the outgrowths of the body-wall, not to the setse) are, moreover, 

 (|uite unrepresented in the related groups of the Archiannelida and Oligochseta. 

 From the facts of their occurrence, therefore, it would appear that they represent a 

 development within the Polychfete stem, and have attained to their full size and 

 differentiation in only a few of the topmost branches of that stem. 



But if the parapodia are a secondary and a comparatively late development, it need 

 hardly be said that the same is not the case with the groups of setfe which accompany 

 them. These are found, in corresponding positions, in certain of the Archiannelida 

 and in all Oligochseta, and represent, therefore, a character of the common ancestor 

 of the Chsetopoda. They are the primary locomotor organs ; in general, the animal 

 works itself along by securing a hold on the substratum by means of its seta3, and 

 then using them as levers. The degree of activity possible to the animal is deter- 

 mined by the size and strength of the setse and tlie degree of development of the 

 muscles which work them, and is thus correlated with the presence and size of the 

 parapodia. The typical positions for the groups of setfe are dorsolateral and ventro- 

 lateral ; it can hardly be doubted that in many earthworms, where the dorsal bundle 

 is placed at the level of or below the horizontal diameter of the body, we have to do 

 with a secondary shifting. 



It follows from the manner in which the setae are employed, that in the material 

 environment of the ancestral Chaetopod there must have been something for the 

 dorsolateral setae to work upon, equally with the ventrolateral ; and this could only he 



