826 PROFESSOR J. STEPHENSON ON 



the case with animals living in crevices, with the dorsal surface covered over in some 

 way ; there would otherwise have been no possibility of the dorsolateral setse obtaining 

 the purchase which is necessary for their effective use. It appears necessary to suppose, 

 therefore, that the ancestors of the Chsetopoda, and the original representatives of both 

 of the larger groups (Oligochseta and Polychgeta), lived in crevices or crawled about 

 under cover — were, to use Willey's term, cryptozoic. 



In the course of evolution the sedentary habit which is associated with the 

 cryptozoic life became in some representatives of the Polychseta more pronounced ; 

 these developed into such tubicolous forms as the Serpulids, whose large feather-like 

 gill-tufts are an adaptation to an extreme form of sedentary existence. In other 

 members of the group the sedentary habit was to a considerable extent, or at certain 

 periods of the life-history, altogether abandoned. In correlation with the assumption 

 of more active habits, the muscular mechanisms of locomotion became better 

 developed ; and the result has been the appearance of distinct parapodia, — projections, 

 largely muscular, of the body-wall, — associated with the setae, the primary locomotor 

 organs : an interesting parallel is seen in certain Rotifers with highly developed 

 locomotor capabilities, where seta- like hairs are accompanied by muscular excrescences 

 of the body-wall resembling parapodia. In extreme cases the lobe-like parapodia 

 themselves are more effective as locomotor organs than the setae, since they can be 

 used as oars in swimming movements through the water ; one may recall Eisig's 

 observations, quoted by Willey (vide ante), that swimming locomotion through the 

 water results from a greater violence of the same movements as are executed in 

 crawling. In Tomopteris the setae have disappeared along with crawling locomotion, 

 and the animals are pelagic. 



To turn for a moment to the Oligochaeta : the earthworms are noteworthy because, 

 as has been mentioned, the dorsolateral group of setae has sunk down to, or below, the 

 horizontal diameter of the body. What the occasion of this was, it is difficult to 

 conjecture ; it does not seem very probable that on taking to a terrestrial existence 

 the evolving earthworms abandoned the cryptozoic mode of life so far as to adopt the 

 habit of creeping mainly on the surface, and not in crevices. But whatever the 

 occasion may have been, it is interesting to observe that the latest modification of setal 

 distribution restores the original condition, in which a number of setae were situated 

 in the dorsal quadrants of the body-circumference, — though with a difference. In 

 many, indeed in the majority, of the Megascolecidae, the most highly differentiated and 

 most variable family of earthworms, the setae have broken loose from their customary 

 positions, and have spread in a more or less complete ring all round each segment, — a 

 perfect adaptation to the cryptozoic habit, to life in crevices. 



It will be admitted that probably no single representative of the present-day 

 Polychaeta comes anywhere near the ancestral form, and that both errant and sedentary 

 branches have diverged, perhaps widely, from their common progenitor. In each 

 branch certain primitive features have been retained ; in each branch certain secondary 



