1902] The Holothurioidea of Northern Norway. 19 



have found in a single specimen is 28.^) Sometimes they are al- 

 together wanting either in the forepart or the hindpart of the body, 

 in single cases perhaps even in both. I know, that the number 

 can go as low as to two, and Såes says, that in some specimens 

 he has vainly looked for wheels. The wheels are much smaller 

 than in the foregoing species, invisible to the naked eye, and do 

 not roughen the skin. All this is also stated by Sårs, who, how- 

 ever, does not give any dimensions. I found the diameter of the 

 wheels to vary between 0.055 and 0.095 mm. The average size of 

 all the wheels varied, in different specimens, between 0.070 and 

 0.080 mm. ; it amounted therefore to Vs of the average diameter of 

 the wheels in the foregoing species. The spokes of the wheels are 

 11 — 16, the teeth on the periphery of the wheels (17— ) 19—26 in 

 number. The number of the spokes is 55 — 60 7o of that of the 

 teeth. 



This species will possibly not be quite so large as M. rmlii. 

 Specimens from Skager Rack, where it seems to reach its greatest 

 dimensions, are up to 6 cm. long. Most of the specimens burst in 

 two, as soon as they are tåken out of the water, and then the body 

 always divides in a certain place, just behind the fore end. M. rmJcii 

 shows, on the contrary, only a little tendency to autotomy. 



Thus I have been able completely to confirm M. Sårs' state- 

 ments regarding this species, and I have also found a further im- 

 portant difference as regards M. rivMi, namely in the building up 

 of the longitudinal muscles. But Sårs seems to me to go rather 

 too far, when he considers the differences great enough to set up 

 a new genus ,,Ol\gotrochus^'. Therefore I have called the species 

 Myriotrochiis vitreits. 



My preceding statements are too sharply opposed to those of 

 Danielssen & Koren (1882 p. 34 — 35), that I should altogether 

 pass them over in silence. They say, that in the specimens, they 

 have examined of Oligotrochus vitreus, the wheels are „as numerous 

 as in Mijriotrochus'^ and suppose that Sars should have „confined 

 his- observations chiefly to the integument of the ventral surface" (!), 

 where the wheels are few or wanting. Even in other respects they 



^) This number is of course not to be considered as an absolute maximum. 

 It agrees, however, -very well with Sars' statement, that he has found the highest 

 number to be 8 — 10 wheels in each interradius („intermuscular space"), if the 

 validity of the same is limited to the three dorsal interradii. In the two ventral, 

 ones I find none or only single wheels. 



