PTEROBRANCHIA OF THE SCOTTISH NATIONAL ANTARCTIC EXPEDITION. 431 



In text-fig. 5 are shown composite figures of buds based upon thirty -five camera lucida 

 drawings. The proboscis is reproduced in the successive figures as gradually increasing 

 in size ; this is correct in the main, but buds with the same number of arms may have 



TEXT-FIG. 5. Figures illustrating the development of the arms or plumes in the buds of Cephalodiscus agglvtinans, for 

 comparison with the figures of C. gilchristi and C. nigrestens in RIDEWOOD, 07 2 , pp. 243 and 236. The lettering is the 

 same as before, namely : 1, 2, 3, 4, 5, the first, second, third, fourth, and fifth arms of the buds ; b, the "body" or 

 metasome of the bud ; b.s., main portion of the proboscis or buccal shield ; m, mouth ; p. I., lower lobe of the proboscis ; 

 po.l., operculum or postoral lamella ; s, stalk of the bud. 



A, aboral view of a bud in which no arms have yet developed ; B, a bud with one pair of arms ; C, a bud with two pairs ; 

 D, a bud with three pairs ; E, a bud with four pairs ; F, a bud with five pairs of arms ; G, the oral surface of the body of a 

 bud in which the lower lobe of the proboscis was turned well upward, and has been cut away. The operculum is seen to 

 be continuous with the last developed (fourth) arm, and is as yet incomplete below the mouth. Approximately x 60. 



the proboscis of different sizes, and there are many buds with three pairs of arms which 

 have a smaller proboscis than others with two pairs or even one pair only. This irregu- 

 larity may be partly accounted for by disparity in the rates of growth of the proboscis 

 and the collar-region, but is to a large extent also to be explained by the fact that the 



(BOY. soc. EDIN. TRAUS., VOL. XLIX., 555.) 



