PTEROBRANCHIA OF THE SCOTTISH NATIONAL ANTARCTIC EXPEDITION. 421 



portion, and possessing the red line which is so curiously constant a feature in the 

 various species of Cephalodiscus. But the epidermis is hopelessly disintegrated in 

 most cases, and it is only in the buds that the proboscis is sufficiently well preserved 

 for its form and proportions to be adequately determined (PI. II. figs. 1-4 ; and 

 text-fig. 5). 



As in other species of Cephalodiscus, the proboscis is a highly mobile organ, and 

 its parts appear in very varying positions in different individuals. It is shown in 

 the sagittal section, fig. 15, and in the frontal sections, figs. 6-10, of PI. II. 



PL II. fig. 14 represents a section which has cut the proboscis-stalk at its narrowest 

 region, and it demonstrates the two proboscis-canals (p-p.) in their usual relation to 

 the pericardium (per.) and the anterior dorsal horns (6.c. 2 a) of the collar-cavity. 



COLLAR-REGION. 



The principal parts of the collar are the arms or plumes, dorsally ; and the 

 operculum or postoral lamella, ventrally. 



(i.) Tentacle-bearing Arms or Plumes. 



The number of these structures is greater than in any of the species previously 

 described, since the full number appears to be nine pairs. This number is, however, 

 not quite constant, as is indicated below. The species which most nearly resemble it 

 in the number of their arms are the three species of Orthoecus (C. densus, C. rants, 

 C. solidus), which were described by ANDERSSON (07), in which the number is stated 

 to be constantly eight pairs ; and C. nigrescens, in which the number is usually 

 seven pairs, although it may rise to eight pairs (RiDEWOOD, 07 1 ). None of the species 

 of Demiothecia which have been previously described have more than six pairs ; and 

 this is the commonest number in the subgenus in question. 



The sections shown in figs. 6-1 1 of PI. II. are chosen to demonstrate that nine pairs 

 of arms may really be present. They have been cut at right angles to the long axis 

 of the zooid, and therefore in a plane which has been described in the Siboga 

 report (HARMER, 05) as " frontal." 



Fig. 6 shows the first seven and the ninth arms of the left side, and the first seven 

 arms of the right side. The study of the entire series of sections shows that all these 

 arms except L. 9 and R. 1 are directed dorsally, in a line prolonging the main axis of the 

 zooid. The eighth and ninth pairs of arms could probably have assumed the same 

 general direction during life ; but they are actually lying in a position which makes 

 their interpretation more difficult than in the case of the first seven pairs. 



In fig. 7, which is the eighth section from that represented in fig. 6, the first 

 seven arms of the left side are connected with one another. The lophophoral region of 

 the collar has, in this section, not yet split up into the separate arms. Four of the 

 membranous partitions which separate the arm-cavities for a short distance at their 



(ROT. soc. EDIN. TEANS., VOL. XLIX., 545.) 



