PTEROBRANCHIA OF THE SCOTTISH NATIONAL ANTARCTIC EXPEDITION. 425 



It results from these figures that 42'5 per cent, of the zooids had nine arms on one 

 side at least ; while 80 per cent, had either eight or nine arms on one side at least. 

 We accordingly draw the conclusion that the typical number of arms, in this species, 

 is nine pairs, but that more than a third of the cases observed had a pair less ; while 

 a smaller percentage had seven, six or even ten arms as the maximum number on 

 one side. 



B 



TEXT-FIG. 4. Arms of Cephalodiscus aijylut inans. A, an arm well extended ; B, an arm in a 

 moderate state of contraction ; C-E, terminal portions of anus. 



The number of tentacles or pinnules on an arm of a full-grown zooid is usually 

 between thirty-five and forty-five pairs, but in some instances the number may be 

 fifty pairs, and in younger zooids the number is less than thirty-five pairs. 



It is difficult to say with certainty whether the dorsal epidermis of the axis of the 

 arm is thick and black in C. agylutimtns, as it is in C. nigrescens, because in most 

 cases the epidermis has disintegrated, and there is more than a suspicion that in those 

 cases in which the epidermis is still present the pigment-granules, which might have 

 been present in them, have become bleached. This suspicion is founded upon the lad 



(ROY. soc. EDIN. TRANS., VOL. XLIX., 549.) 



