Systematic guide to the genera and species of Rhizocephala. with anatomical diagnoses. 117 



Remarks. Si/Ion appears to be a characteristically northern genns, its head quarters 

 being apparently in the Norwegian and Greenlandic Seas, while extending south as far as 

 Puget Sound on the American coast. 



This genus has been studied by Michael Sars (10) and later by Hoek (21). Sars 

 wrote before the anatomy of the Rhizocephala was well understood, and his views are very 

 incorrect, as he takes the eggs in the mantle cavity for the ovary, the whole visceral mass 

 for a testis, the colleteric gland for the apex of the testis, and the two mantle openings viewed 

 from inside as the openings of the vasa deferentia. 



Hoek's views are far more correct, and I disagree with only one part of his description, 

 though that is of fundamental importance, and relates to the presence or absence of a testis. 

 I judge from Hoek's statement that he himself is not over confident of the nature of the 

 body which he figures as a testis, for he says: "only in one of the specimens did I observe 

 anything that could be considered as a testis" p. 926, and I must confess that his figure of 

 this organ does not resemble the testis of any other Rhizocephala. 



I have prepared serial sections through the visceral mass of four specimens of Sj/lon, 

 kindly put at my disposal by Dr. H. J. Hansen of Kjobenhavn and by Professor D'Arcy 

 Thompson of Dublin; two of these specimens were in excellent preservation and gave a per- 

 fect series of sections, while the other two were sufficiently good to serve as valuable checks. 

 In none of them was a trace of testis to be observed, and I could hardly miss this organ now 

 after studying it in the most various forms of Rhizocephala both by dissection and sections 

 for about two years. I am therefore confident that Sylon is structurally a female and not a 

 hermaphrodite and I feel justified in drawing the conclusions on this head given in Chapter 2 

 of this work. I would also say a word on the nervous system of this genus; the ganglion, 

 situated normally near the mesentery and in close proximity to the mantle openings, is very 

 much larger proportionately to the rest of the body, and the nuclei of the cells preserve their 

 staining capacity far more than in other Rhizocephala. (See Plate 8 fig. 4 gn.) 



I have been able to confirm both by observations on the whole animal and by sections 

 the presence of two small mantle openings, as shown in Plate 8 fig. 4, in place of the un- 

 paired opening usually found in Rhizocephala. 



Genus ClistosacCUS (Lilljeborg [4]). 

 (See Plate 8 figs. 5—3.) 



Diagnosis. External body of adult red. 

 Roots colourless (?), compact. 



Mantle and visceral mass highly muscular, mantle smooth and unconstricted. 

 Mantle opening absent. 

 Mesentery very thin, running in long axis of body on surface of fixation, parallel to 



long axis of host. 

 Body irregularly oblong, uncompressed. 



