18 ON THE ECHINODERMATA OF THE 



with the Greenland form of Steenstrup's species. In one characteristic especially do 

 they correspond with, and by the same differ from, the ordinary arctic forms of M. EinJcii. 

 In this latter the spicules are attached to the skin by a kind of stalk or prolongation 

 of the cuticular membrane, whilst in 0. vitreus and Dr. Theel's specimens of Myrio- 

 trochus it is particularly mentioned that these are embedded in the skin itself. So 

 marked is the feature, that amongst all his material Dr. Theel has never seen an indi- 

 cation of the " stalk ;" and he is hence led to question the accuracy of Steenstrup's 

 description in this respect, and to take, at the same time (quite erroneously, it would 

 seem to us) certain words used by Prof. Huxley when describing Chirodota brevis as 

 supporting the same conclusion (Theel, I. <?. p. 4 ; Huxley, I. c. p. ccxii). Now it is 

 very remarkable that, on the other hand, every specimen which we have examined, 

 whether from Greenland or the extreme north, has the spicules attached by membranous 

 stalks, and that this is a character which is at once apparent on examining them with 

 no greater magnifying-power than a simple hand-glass. It is, in all probability, true 

 that the spicules could be drawn more or less tightly down to the surface, as Steenstrup 

 has already suggested, from the nature of their attachment (Steenstrup, I. c. p. 58) ; 

 and in some rare cases, where the spicules have been abraded, there seem to be little 

 cavities left in their place in the membrane ; but we are disposed to believe that this 

 results rather from the change which has taken place in the tissue around the closely 

 appressed spicule whilst in spirit, than from a natural depression existing during life. 



Distribution. 



a. Northward of Smith Sound : Discovery Bay, lat. 81 41' N. (Naress Exped.), 

 the most northern locality on record ; Port Foulke, lat. 78^ N. (Stimpson) ; lat. 69 31', 

 100 fms. (' Valorous' Exped.). 



In Greenland : Godhavn ; Omenak ; Arksut, 10 fms., mud (Llitken) ; Germania 

 Harbour in E. Greenland, 2 fms. (Moelius). 



b. North of American Continent : Assistance Bay (Penny's Exped.) ; Labrador 

 (Packard, fide Verrill). 



c. North of European Continent: Spitzbergen (Lutken); Novaya Zemlya, 

 2-70 fms., clay ; Vaigatch ; Kara Sea, 11-60 fms., sandy clay ; Sea of Murrnan (Theel). 



Description of the Illustrations of this Species on Plate I. 



Fig. 20. The animal : natural size. 



21. One of the spicules : magnified. 



22. Sketch of the internal organs, showing the relative proportions of the alimen- 



tary canal, the Polian vesicle, and the generative tubes. 



23. Portion of the mouth-ring : magnified. 



24. Underside of mouth-ring, with parts attached, seen from below, showing the 



dilatations of the oral water-vessel. 



