922 



THE VOYAGE OF H.M.S. CHALLENGER. 



The following list gives the different cases in which species of Sylon have hitherto 

 been observed : — 



The specimen of Sjnrontocaris spinus on which the parasite was found had a length 

 of 37 mm. It was attached to the third segment of the abdomen. According to 

 M. Sars, Sylon hippolytes is also attached to the third, and Sylon pandali to the first 

 abdominal segment of its host. According to my own observations, Hippolyte pusiola 

 likewise bears its Sylon on the third segment of the abdomen. 



In the case of Spirontocaris spinus, as shown in PL CXLIX. fig. 1, the parasite is 

 attached by a considerable part of its surface, the attached part being circular and having 

 a diameter about half as long as the longest axis of the parasite. The body- wall of the 

 shrimp and of the Sylon almost imperceptibly pass into one another ; when separating 

 the parasite its chitinous covering was found to have a yellow-coloured thickening, of the 

 shape of a ring, round the place of attachment. 



The shape of the parasite is oval, 1 its long axis running nearly but not quite parallel 

 with that of the Shrimp. If we apply the term poles to the extremities of the longest 

 axis, then the anterior pole is situated at a somewhat greater distance from the ring of 

 attachment than the posterior pole. In the species of Sylon found upon Hippolyte 

 pusiola, and which I will call Sylon schneideri, not only is the greater part of the body 

 of the parasite situated in front of the base of attachment, but the anterior pole is at a 

 considerably greater distance from the surface of the host than the posterior pole. 

 The greatest diameter of Sylon challengeri measured about 4 mm., and the two other 

 axes only measured 3'16 and 2'6 mm. Taking the plane of the two other axes as 

 perpendicular to the direction of the longest axis, the one second in length (3*16 mm.) is 

 perpendicular, or nearly so, to the surface of the Shrimp ; the shortest of the three is the 

 one that runs from the. right to the left side of the body of the parasite. 



1 The figures of the parasite on PI. CVI. figs. 5i, 10, represent it as spherical ; but this is not quite ex-act. Fig. 10 

 also shows the parasite as being attached by means of a short but distinct peduncle, but this is not the case. 



