426 DR S. F. HARMER AND DR W. G. RIDEWOOD ON THE 



that in the Discovery material some of the pieces of C. nigrescens were carefully fixed 

 in Perenyi's fluid and some in picric acid solution, but most of the material was 

 preserved in 5 per cent, formalin ; and in slides made from the former material (fixed 

 in Perenyi's fluid), the pigment is strongly marked (RIDEWOOD, 07 1 , pi. v. fig. 28), 

 whereas in sections prepared from the formalin-preserved material, the thick dorsal 

 epidermis is not darker than the other parts of the section. The material of 

 C. agglutinans was preserved in alcohol and not in formalin, it is true, but the 

 preservation is not good, and the appearance of the large cells on the dorsal surface 

 of the axis of the arms seen in paraffin-prepared serial sections is remarkably like that 

 in the sections of the formalin-preserved zooids of C. nigrescens. 



The dorsal epidermis of the arms of C. agglutinans is least disintegrated in contracted 

 arms, and in the arms of fairly young zooids having but twenty-five to thirty-five pairs 

 of tentacles. Text-fig. 4, A shows the appearance of a well -extended arm, and B an arm 

 in a moderate state of contraction. The terminal part of the arm has no end-bulb 

 with highly refractive beads such as distinguish the arms of C. dodecalophus (M'lNTOSH, 

 87, pi. iv. fig. 1 ; pi. v. fig. 1 ; and RIDEWOOD, 07 1 , text-fig. 1, p. 4) and C. hodgsoni 

 (07 1 , pi. v. fig. 32). In extended arms the extremity is bluntly pointed ; in contracted 

 arms it is rounded and even hemispherical. Some terminations better preserved than 

 usual are shown in text-fig. 4, C-E (cf. C. nigrescens, 07 1 , PI. v. figs. 23-27). 



(ii.) Opercitlum or Postered Lamella. 



The interpretation of the sections shown in figs. 8-11 is at first sight by no 

 means easy, so far as the operculum is concerned. The study of the plasticine 

 reconstruction has, however, enabled us to come to a clear conclusion as to its general 

 form. It is seen to be deeply emarginate in the middle line, and the median sagittal 

 sections examined thus show only a feebly developed lower lip (e.g. fig. 15, op.). 

 On either side of the mouth, however, the operculum is produced into a large lateral 

 lobe, a great part of which is free. The comparison of this series with other series 

 of sections, and with what is known of other species of Cephalodiscus, shows that the 

 operculum is a highly mobile organ, the parts of which can assume very different 

 positions at the will of the zooid. In the specimen under consideration the right 

 lobe (op.r.) is directed in the main dorsally (figs. 10-8), while the left lobe is sharply 

 reflected ventrally, close to its origin from the collar. Figs. 10 and 9 cut this left 

 lobe at or near the point where the flexure takes place. A consideration of fig. 9 

 will show that if the lobe of the left operculum (op.l.) nearer to the proboscis be 

 imagined to have been unbent, so as to pass in a dorsal direction as a prolongation of 

 the part of the operculum of the same side which is nearer the central nervous 

 system, it would not have appeared in fig. 9 ; and the two sides of the section would 

 have been fundamentally similar. If the zooid had died with the left lobe in the 

 position indicated, both lobes would have appeared in fig. 8 in the position actually 



(ROY. SOC. EDIN. TRA.NS., VOL. XLIX., 550.) 



