ARCTIC SEA TO THE WEST OF GREENLAND. 17 



The water-vascular ring is broad and conspicuous ; and although only one Polian 

 vesicle is developed, which is large and heart-shaped, a number of dilatations occur at 

 intervals on the ring, which are suggestive of incipient or aborted vesicles (cf. fig. 24). 



The generative organs consist of two groups of dichotomosing tubes which unite in 

 a single duct, that makes its exit outside the tentacular ring. The sexes are distinct 

 and readily distinguishable : in the female the tubes are shorter but very much broader, 

 and the large-sized ova can generally be seen within ; in the male, on the other hand, 

 the tubes are longer and thinner as well as more numerous. In neither sex do these 

 tubes, as a rule, extend beyond the anterior third of the body. 



Premature Form. We are unable to speak from personal observation of the 

 growth-phases of Myriotrochus, as all the specimens at our disposal have been more or 

 less perfectly developed. Dr. Hj. Theel* mentions an individual 10 millims. in length, 

 presumably referable to this species, as the smallest he had examined. He states that 

 " the skin contained a great quantity of the wheels in different stages of development, 

 some even perfectly formed, although amongst the whole it was scarcely possible to 

 find two that were identical. Many of the spicules had not more than six or ten radii, 

 but bore twice that number, or even more, of the denticles. This example, which was 

 a female, contained, notwithstanding its small size, fully developed eggs " (loc. tit.). 



Variations. On the whole, Myriotrochus Rinkii is a species that seems to 

 undergo only very slight variation in form; the main difference which we have 

 remarked occurs in the number of spicules that are present on the cuticle. For 

 instance, in some examples the whole of the three dorsal interradia are closely filled 

 with spicules, and a fair sprinkling is distributed over the ventral area as well ; whilst, 

 on the other hand, specimens are not rare in which these are confined to the median 

 dorsal interradial area, with, perhaps, just a few on the edge of the adjoining areas, 

 the ventral portion being quite bare, except three or four isolated wheels at the anterior 

 extremity ; and there are yet others even more sparingly provided, which have only a 

 few isolated spicules at the extremities of the dorsal surface. Respecting the above- 

 named variable character, we are bound to confess that we are at present unable to 

 assign even a locational value to these modifications, since both extremes occur together 

 in the material received from the most northern collecting-station recorded. 



With these facts before us, it would seem that the characters upon which Sars 

 founded his genus Oligotrochus are all to be found in Myriotrochits ; and consequently 

 no grounds exist on which that genus can be longer maintained. Indeed we have for 

 some time been of opinion that even the species 0. vitreus, Sars, could only be regarded 

 as a variety comprehended within the form of Myriotrochus Rinkii; but a recent paper 

 of Dr. Theel's, and a careful comparison of Sars's description and figures with the 

 material we have had before us, lead to the conclusion that Oligotrochus vitreus may 

 probably rank as a species of Myriotrochus allied to M. EinMi ; and we are also further 

 inclined to think, from the description given, that Dr. Theel's specimens from Novaya 

 Zemlya and the sea of Kara would accord more nearly with Myriotrochus vitreus than 



Nova Acta Keg. Soc. Sc. Upsal. ser. iii. 1877, no. xvii. p. 6. 



