CTENOPHORA. 



those represented, evidently in natural size, on the PI. I of L,. Agassiz' memoir. According to Van- 

 hoffen they may reach a size of 25 mm in diameter, and Evans and Ashworth 1 ) even record a 

 lenght of 30 mn) ; so large I have not observed them myself. 



In a few specimens I have observed a parasitic Nematod, lying within the jelly; in one case 

 the parasite had wound itself round the tentacle basis. All the specimens of the worm being imma- 

 ture, it seems evident that it does not reach its full development in the Pleurobrachia, but in some 

 other animal, which preys upon the latter. I can give some information hereof. In the collection of 

 the Copenhagen Museum there is a specimen of a Pleurobrachia found in the stomach of Cyclopterus 

 lumpus (from the North Sea, Capt Soiling); this fact, combined with the observations of O. Fabri- 

 cius (Fauna groenlandica, p. 363) that at Greenland Mertensia ovum is eaten by the same fish, seems to 

 leave no doubt that Cyclopterus lumpus really preys upon the Pleurobrachia; it may then not seem 

 improbable that the Nematod occurring in the Pleurobrachia is the young of one of those Nematods 

 found as parasites in the Cyclopterus, viz. Ascaris succisa Rud. and Agamonema capsularia Dies., and 

 of these two it would evidently be the latter, the diagnosis (Die sing, Systema Helminthum II, p. 116) 

 "extremitate caudali obtuse cum acumine" being quite in accordance with the specimens observed in 

 the Pleurobrachia. Also the size is in accordance, the specimens found being ca. 1-5 cm , while the size 

 of Agamonema is stated by Diesing to be '/a i". 



The occurrence of this Nematod in Pleurobrachia was already observed by Forbes, who in his 

 paper "On two British Species of Cydippe" 2 ) (Ann. Nat. Hist. III. 1839 p. 148) says that "imbedded in 

 the substance of one of these animals, near the stomach, is a remarkable parasitic worm, in shape 

 resembling a Filaria" . 



Besides the Nematod, I have found a small Trematod in the Pleurobrachia, in the pharynx or 

 in the jelly of the body; it may occur in considerable numbers in the same specimen and is much 

 more common than the Nematod. It may be suggested that it is the young of one of the distomes 

 living in Cyclopterus, which of them I do not venture to say - - the more so, as there is also the pos- 

 sibility, that the final host of these Trematods is another animal. I have been informed by Dr. 

 C. G. Joh. Petersen, director of the Danish Biological Station, that he has sometimes found specimens 

 of Acanthias vulgaris having the stomach full of Pleurobrachia. It is then, of course, also 

 possible that Acanthias is the final host of the Nematod found in Pleurobrachia. It is worth 

 pointing out that also in Acanthias the Nematod Agamonema capsularia occurs. On the other hand 

 the same Nematod occurs in a considerable number of fishes (e. g. the cod, salmon) so that it must be 

 supposed that the Pleurobrachia is not the only first host of this parasite. 



In the material of the "Ingolf" Expedition there are no specimens which can with any certainty 

 be referred to this species. 



The distribution of Plc-urobrachui pileus appears to be very wide, perhaps cosmopolitan. It is 

 common over the whole of the North Atlantic, both at the European arid American Coasts. In the 

 Danish seas it is common everywhere, in the North Sea, Skagerak and Kattegat. In the Baltic it may 



1 ) Win. Evans and I. H. Ashworth: Some Medusae and Ctenophores from the Firth of Forth. Proc. R. Phys. 

 Soc. Edinburgh. XVII. 1909. p. 308. 



2 ) The paper appears to have been overlooked by the recent authors on Ctenophora. The two species described, but 

 not named, by Forbes are evidently both Pleurobrachia pileus. 



