746 PROFESSOR J. STEPHENSON ON 



discussion ; and it is therefore of interest to speculate as to the reasons for the remark- 

 able exception in the case of Chsetogaster. 



Supposing, as has been done throughout, that the phenomena of antiperistalsis and 

 ascending ciliary action are respiratory in purpose, their absence in this genus might 

 be due to the present or past occurrence of conditions of life which do not or did not 

 permit of this means of respiration — that is, to life in a medium other than water. Two 

 such possibilities present themselves — a terrestrial, or a parasitic existence. While 

 there is no reason for thinking that the present-day forms are descended from terrestrial 

 ancestors, there are, on the other hand, certain facts which seem to point towards a 

 parasitic habit in the past. 



In the first place, one species, C. limnxi, may actually be found as an endoparasite 

 in the liver of certain Pulmonata, while others of the same species live on the surface 

 of the same water-snails. Other species of the genus, though not parasitic, may live 

 as commensals; thus C. bengalensis was usually found by Annandale (1) clinging by 

 means of a posterior sucker to the external surface of the body or to the edge of the shell 

 of water-snails {Lwnnseus, Limnophysa, etc.), sometimes entirely withdrawing within 

 the shell when disturbed ; this species may also, however, be free-living. The same 

 observer (2) found C spongilla} living in Spongilla carteri, and again on the surface 

 of a variety of Plumatella 7'epens ; while another species of Chwtogaster also lives on 

 the same Plumatella. 



Secondly, the carnivorous habit is characteristic of many, perhaps of all, members 

 of the genus. I found (46) in the alimentary canal of C. orientalis, small Crustacea, 

 rotifers, small nematodes, Paramsecium and other Ciliata. Setse were often found in 

 the crop, probably belonging to specimens of Nais communis which had been digested. 

 On one occasion I found several individuals exploring with their mouths the surface of • 

 a dead and decomposing fly ; on another, I observed for some time the efforts of a 

 Chsetogaster to engulf in its distensible pharynx a Daphnid, which was, however, 

 far too large to be thus disposed of. C. bengalensis (Annandale, loc. cit.) devours 

 small Crustacea ; C. spongillw eats the dead and decaying parts of the sponge in 

 which it lives ; a third species observed by Annandale feeds on Protozoa ( Vorticella, 

 Epistylis, Stentor, etc.) ; Annandale, however (I), mentions having had an English 

 species sent to him in which the food probably consists of diatoms and the like. 

 Since the Naididse in general are vegetable feeders, it seems not unlikely that the 

 carnivorous habit of the genus Chsetogaste?' may be a remnant of a previous parasitic 

 mode of life. 



Under this head, along with the generally carnivorous habit, may be mentioned the 

 modification of the anterior, or both anterior and posterior (Annandale, 1), extremities 

 of the body in some species to form suckers ; and the powerful pharynx, much larger, 

 and attached to the body- wall by much more numerous and regularly radiating 

 muscular strands, than in other Naididse. In the other Naididse also, the mouth is a 

 transverse slit on the ventral surface, some distance behind the anterior tip of the body ; 



