256 Prof. A. C. Haddon on Larval Actintm 



XXXIII. — Researches at the St. Andrews Marine Labora- 

 tory [under the Fishery Board for Scotland) . — On 

 Larval Actinice parasitic on Hydromedusce at St. Andreivs. 

 By Alfeed C. Haddon, M.A. (Cantab.), M.R.I. A., Pro- 

 fessor of Zoology, Royal College of Science, Dublin. 



Several larval Actinige, together with the Leptoraedusse on 

 which they were found, were sent to me by Prof. W. C. 

 M'Intosh in the month of February in the present year. None 

 of the specimens were sufficiently advanced for me to be able 

 to say with certainty to what genus or species they belong, but 

 Prof. Mcintosh has kindly promised to get me more specimens, 

 and I hope that with increased material I may be able to do 

 better justice to the beautifully preserved specimens which 

 my friend has entrusted to me. 



I have previously referred (1) * to the recorded instances of 

 larval Actinige being found as ectoparasites of Medusae, one 

 of the first records being that of Dr. T. Strethill Wright (2) 

 from the Firth of Forth. The Actinige attach themselves to 

 the upper or under surface of the bell^ or to the margin^ or to 

 the gastric region of the Medusge. 



The Actinige from St. Andrews are of slightly different 

 ages ; the youngest exactly corresponds to the stage of Hal- 

 camjja chrysanthellum described and figured by me {I. c, pL xi. 

 figs. 1-5) . At tliis time there are eight larger mesenteries 

 which correspond in position and in the arrangement of their 

 longitudinal retractor muscles with the eight mesenteries of 

 Edioardsia, but in addition there are four smaller mesenteries 

 which form the complemcntal pairs to the four lateral larger 

 mesenteries. There are also only eight short tentacles, which 

 are arranged in two groups of three, the two others being 

 opposite one another and between the two groups. The ten- 

 tacles arise from the chambers between the eiglit large mesen- 

 teries. The inter-mesenteric chambers (entoccels) of each 

 lateral pair of mesenteries are not prolonged into tentacles, 

 and it is the four gaps, corresponding to these chambers, which 

 cause the arrangement of the tentacles noted above. It will 

 be seen that the central tentacle of each of the groups of tliree 

 corresponds with the entocoel between each pair of directive 

 mesenteries. As in the parasitic larva of Halcampa chrysan- 

 thellum there is a single deep oesophageal groove ; the grooved 

 end of the oesophagus likewise extends a short distance below 

 the normal lower border of the oesophagus. 



* See list of papers at end of this article. 



