88 DISCOVERY REPORTS 



smallest 1-5 cm. long. The size of the largest animal is greater than any yet recorded for this species, 

 unless we accept A. plicata Kott as a synonym, Kott having recorded specimens of that species up to 

 29 cm. in length (see below for a discussion of A. plicata). 



Dorsal tubercle. The species is distinguished mainly by the very complex form of the opening 

 of the dorsal tubercle. The large specimens in the ' Discovery ' collection show this feature but in 

 smaller ones the opening is much simpler, and a series of increasing complexity can be seen (Text- 

 fig. 26C-E). 



Oral tentacles and branchial bars. The numbers of oral tentacles and of longitudinal branchial 

 bars are given in Table 26, for specimens of different sizes. 



Table 26 



These figures give the total numbers of tentacles, including the smallest visible ones. If the larger 

 and more conspicuous tentacles only are counted, the numbers would be reduced by one-third to 

 one half. The number of tentacles does not increase steadily with growth of the body ; in fact there is 

 a tendency for large animals to lose tentacles. 



Branchial sac. The wall of the branchial sac is plicate, as noted by Herdman (1882). I have found 

 that in large specimens the plication is complex, as the walls of each fold are thrown into a series of 

 lesser folds, as described by Kott (1954) in her species A. plicata. Correlated with this folding there 

 has developed immediately external to the perforated wall, a flat sheet of tissue extending over the 

 whole outer surface of the wall. This sheet represents an expansion of the outer part of the transverse 

 bars, and serves to support the folded stigmatic wall. The sheet is pierced by transverse rows of slit- 

 like openings by which alone the water may escape from the branchial cavity to the surrounding peri- 

 branchial cavities. 



The papillae of the branchial sac are large, and in some places intermediate papillae may be seen, 

 but these seem usually to be associated with the growth of new transverse bars. 



Remarks. A. translucida is readily distinguished by its dorsal tubercle from other Antarctic and 

 Subantarctic species except A. plicata Kott (1954) in which a similar tubercle has been described. 

 Kott states that this species is distinguished from A. translucida by (1) the branchial wall with primary 

 and secondary folds, (2) the atrial siphon one-third to one half of the body length from the oral 

 siphon, and (3) the absence of secondary branchial papillae. 



I believe that the branchial wall of A. translucida is essentially the same as that described for 

 A. plicata. The type specimen of A. translucida, which Herdman described and illustrated, was only 

 2-2 cm. in length, and a specimen of this small size might be expected to have a simple type of folding 

 in the branchial sac. The 'Discovery ' specimen of 2-9 cm. length has only slight folding of the wall, 

 and the smallest specimen available, of length 1-5 cm., shows no folding at all. There is, therefore, a 

 gradual increase in complexity with increasing age and size. 



The second feature used to distinguish A. plicata, the distance of the atrial siphon from the anterior 

 end of the body, does not in fact separate the two species, in each of which the distance is between 

 one-third and one half of the body length. 



