CHARACTEE8 OF SPECIFIC VALUE. 3 



DIAGNOSIS OF A NEW SPECIES OF CEPHALODISCUS (C.hodgsoni}. 



The principal characters of C. hodgsoni are as follows : C. liodgsoni n.sp. 

 (fig. 1, plate 2) tubarium, an irregularly branching tube, with lumen varying in size, 

 but with inner surface smooth, and not with partial septa and trabeculae ; ostia oval, 

 about 3'3 by 2' 3 mm., with four or five long radiating spines, simple or forked; 

 polypides colourless or nearly so, with no black or brown pigment ; males, females, and 

 hermaphrodites (with one ovary and one testis) occurring mixed in the same colony, 

 indistinguishable externally ; plumes twelve, each with an end bulb, in the epidermal 

 cells of which are refractive colourless beads ; free eggs found within the cavity of the 

 tubarium measure about '45 mm. across. 



CONSIDERATION OF THE CHARACTERS SERVICEABLE FOR THE 

 DISCRIMINATION OF THE SPECIES OF CEPHALODISCUS. 



In reviewing the six species of Cephalodiseus of which diagnoses have up to the 

 present been published,* one cannot fail to be struck by the close agreement in general 

 structure that exists between the polypides of the various species. Were it not that 

 the males of C. sibogae have a reduced alimentary canal, and the plumes reduced to 

 two, one might draw up a general description of a polypide which with very slight 

 alterations, having reference to the size of the body as a whole, the proportions of the 

 buccal shield, stolon and stomach, the number and shape of the plumes, and the degree 

 of pigmentation of the surface, might be made to apply to the polypides of any 

 species. The details of the structure of the notochord, gill-slits, proboscis canals, 

 collar canals, and gonads are almost identical in all. It becomes necessary, therefore, 

 to look mainly to the tubarium for features which shall serve to distinguish one species 

 from another. 



The extreme mobility of the buccal shield renders a comparison of the shields of 

 the various species a matter of practical difficulty, and one can only utilise this organ 

 for taxonomic purposes by taking an average of measurements of a large number of 

 shields of each species. The structure of the shield is essentially the same in all ; the 

 curved red line is constant, and the posterior lobe is thinner than the anterior. 



The number of plumes is not constant in polypides of the same species, but the 

 degree of variation is not the same in all species. In Cephalodiseus dodecalophus, 

 C. levinseni, and C. hodgsoni the number is almost invariably twelve ; in C, nigrescens, 

 on the other hand, the average number is fourteen, but there may be as few as twelve 

 or as many as sixteen. The number of plumes in C. gracilis is stated by Harmer to be 

 ten ; the plumes of C. sibogae are eight in neuter individuals and two in the males. 



* The description of Dr. Andersson's specimens has not reached me at the time of writing, and the new 

 species of Cephalodiseus dredged by Dr. Gilchrist in the Cape Seas has not yet been described. 



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