424 



ON DIDYMORCHIS, A RHABDOCOELE TURBEL- 



LARIAN INHABITING THE BRANCHIAL CAVITIES 



OF NEW ZEALAND CRAYFISHES. 



By Professor William A. Haswell, M.A., D.Sc, F.R.S. 



(Plates xx.-xxi.) 



The Rhabclocoele Turbellarian with which this paper deals was 

 found by me in the branchial chambers of Paranephfops neo- 

 wlanicus from streams in the Province of Otago, New Zealand. 

 It is, so far as I have had the opportunity of observing, practically 

 an invariable companion of the Crayfish, having been found, though 

 never occurring in large numbers, in all the specimens examined; 

 and it appears to pass its entire existence in the interior of the 

 branchial chambers of its host. My attention was first directed 

 to it owing to some resemblances to the Temnocephalere, for 

 allies of which I was then searching; and some of these resem- 

 blances are of sufficient importance to justify the conclusion that 

 we have here probably the nearest known Rhabdocoele relative 

 of the group in question. 



The animal (PI. xx., fig. 1) is about one millimetre in length 

 and less than a third of a millimetre in greatest breadth. It is 

 dorso-ventrally compressed, flat below, slightly convex above. 

 The breadth is nearly uniform throughout when the animal is 

 moderately extended, slightly greater towards the middle of the 

 body. The anterior margin is nearly transverse, but moderately 

 convex: the posterior is more strongly convex, and is thickened 

 owing to the strong development of the muscular layers 



There is no pigment anywhere except in the eyes. The latter 

 are situated immediately in front of the pharynx, about a fourth 

 of the total length behind the anterior margin, and are separated 

 from one another by a space which is equal to about one-half of 

 the breadth of the body in that region. 



