920 THE VOYAGE OF H.M.S. CHALLENGER 



species were known to him. Of the third genus, Sylon, Kroyer proposed only one species, 

 though the different specimens show considerable variation in shape and size. It seems 

 to occur on the genus Hippolyte only. No description of the genus Sylon is given in 

 this note ; and Kroyer' s death in 1870 occurred before the paper, in which he intended 

 to give a full description of the different species and genera, was published. With regard 

 to Sylon the only things we learn from his note of 1855 are that its metamorphosis is 

 much like that of Pachybdella and Peltogaster, and that he believes it to be the only 

 genus of the group in which a kind of vascular system occurs. 



In 1870 G. 0. Sars published 1 the second part of his father's Bidrag til Kundskab 

 om Christianiafjordens Fauna, with the aid of the manuscript left by his father, 

 Dr. Michael Sars, who died in 1869. The same memoir was also published separately. 2 



In this paper a description is for the first time given (pp. 41-48) of the genus Sylon, 

 Kroyer, and of two species belonging to it. The one is Sylon hippolytes (Kroyer), 

 most probably the same species that Kroyer observed ; it was found on the under 

 side of the abdomen of Hippolyte securifrons, Norman, which was taken at a depth of 40 

 to 60 fathoms in Storemedet, and at a depth of 100 to 120 fathoms in the Rddtangdybet. 

 M. Sars points out that the same species occurs attached to a specimen of Hippolyte 

 p>olaris, Sabine, which Daniellsen obtained in Hardangarfjorden at a depth of 250 

 fathoms. The other species described is Sylon pandali, M. Sars, a parasite of Pandalus 

 brevirostris, which lives at a depth of 25 to 60 fathoms " in freto Dr0bachiensi." 

 Both species are figured and a fairly full description is given, the only one hitherto 

 published. 



The diagnosis which M. Sars proposes for the genus Sylon is as follows : — 



" Corpus sacciforme, ovatum, subteres, cute (pallio) pellucida seel firma vestitum. Os 

 vel apertura suctoria in organo adfigendi acetabuliformi, annulo corneo cincto, in latere 

 inferiore corporis situm, ubi in posteriore parte aperturae (genitales) binge parvse circulares 

 beantes, symetrice positse, cavitatem intrapallialem aperientes, adsunt. Genitalia bi- 

 sexualia : ovarium ramosum, in sacco magno maxim am partem cavitatis interpallialis 

 explente inclusum ; testiculus parvus ovatus, in posteriore parte ventrali hujus cavitatis 

 situs." 



At the end of his description of the two species, Sars points out the differences 

 existing between Sylon and the other known members of the family Peltogastridse, 

 established by Lilljeborg. Sylon differs from Peltogaster in not having an aperture at 

 the anterior extremity of the body, and also in having only a single testis ; from Apeltes 

 it differs both by the absence of the anterior aperture and of the short tube at the 

 hindermost extremity of the body, and by the presence of a well-developed organ for its 

 attachment to the host, with a mouth in the centre. Sylon also differs from both 

 by the shorter form of the body, in whieh respect it rather resembles Clistosaccus of 



1 Nyt Mag. f. Naturvid., vol. xv. 2 Ckristiania, Johan Dalil, 1870. 



