322 Transactions. — Zoology. 



any floating matter in the water would naturally tend to settle 

 in these spaces, and would interfere with the proper use of the 

 eyes ; and there seems little doubt that the fifth pair of legs 

 are of advantage to the animal in keeping these spaces clear, 

 and in preventing parasites lodging therein. 



It is comparatively seldom that one has an opportunity of 

 observing the habits of marine animals, and the use that is made 

 in this case of a pair of appendages that have all the appearance 

 of becoming vestigial is perhaps worth placing on record. 



EXPLANATION OF PLATE XIX. 



Fig. 1. Grimothea gregaria, fifth thoracic leg ( X about 15). 



Fig. 2. Grimothea gregaria, extremity of the same (more highly magnified). 



Art. XXVI. — On the Occurrence of a Species of Cercaria in the 



Cockle. 



By Chas. Chilton, M.A., D.Sc, F.L.S., Professor of Biology at 



Canterbury College. 



[Bead before the Philosophical Institute of Canterbury, 30th November, 1904.] 



Plate XIX. 



During the month of November, 1904, I spent a short holiday 

 at the Portobello Fish-hatchery and Biological Station, and on 

 the morning after my arrival the curator, Mr. T. Anderton, 

 brought me a cockle (Chione stutchburyi) evidently infested with 

 some parasite. On examination this proved to be the sporocysts 

 of some species of Distomum containing numerous Cercaria, in 

 most cases just ready to escape from the sporocyst. Subsequently 

 the sporocysts were found in two or three other specimens of the 

 cockle, but, though numerous in those specimens in which they 

 were found, they were not often met with — certainly not in 1 per 

 cent, of the cockles examined. 



Similar Cercaria? have been long known to occur in various 

 molluscs in Europe, but so far as I am aware the only forms 

 hitherto recorded from New Zealand are the two described by 

 Professor Haswell from Mytilus latus ;* and, as my specimens 

 differ from these and from all the others that I can find descrip- 

 tions of, I give here a brief description of its general form, mode 

 of occurrence, &c. It appears to come very near to Cercaria 

 villoti, Monticelli (= C. setifera, Villot),f but differs from that 



*Proc. Linn. So. . N.S.W.. 1902, p. 497. 



t See Korscheldt and Beider, "Embryology of Invertebrates," part i. 

 (English edition), p. I*ti; and Villot, "Annales des Sciences Xaturelles," 

 Zool., viii., p. 33. (I am indebted to Professor Benham for these references. 



