270 



DISCOVERY REPORTS 



Genus Tetrastemma, Ehrenberg 



Tetrastemma esbenseni, n.sp. (Plate XVI, figs. 4, 23 ; Figs. 46, 47). 



This species is not a common one. It occurred as follows: 



7. iii. 26. King Edward Cove, South Georgia, 1 (N 18). 



9. iii. 26. Maiviken, under stones, 7 (N 18). 



St. 39. 25. iii. 26. OTL, 179-235 m. 2 (N 44) ; N 4-T, 20 (N 23, N 93, N 1 12). 



St. 123. 15. xii. 26. OTL, 230-250 m. 1 (N 18). 



The lengths and corresponding breadths were: 30-0, 07 mm.; 90-0, o-8mm.; 

 22-0, 07 mm.; 53-0, 1-3 mm. 



The body is thin, soft and round in section, tapering at the head and tail. The head is 

 pointed and there is no neck between it and the body. There is a chevron groove be- 

 hind the head, complete ventrally. The eyespots are arranged in four groups at the 

 corners of a rectangle, the number in each group varying from one to six, the anterior 

 groups usually containing more than the posterior. The colour is light yellow or yellowish 

 red. The gut can usually be seen through the body wall. In the specimens which were 

 separated as N 44 the vascular system was more engorged than usual and no eyes were 

 seen (Plate XVI, fig. 23), but they were identified with the other forms from the 

 sections. 



Form and colour of spirit specimens. The body is contracted and cylindrical (lengths 

 18 and 15 mm. with breadths of 1-4 and i-omm.). All colour is bleached and no 

 eyes are visible. On clearing in cedar oil one 

 specimen possessed the usual two pairs of eyes 

 imperfectly formed, another had four large eyes 

 and several small perfect brown cup-shaped eyes 

 close to them. Two more variations are shown in 

 Fig. 46. The proboscis is always small and thin. 

 The armature consists of a main stylet 0-065 mm - 

 long on a base 0-083 mm - an d a curiously irregular 

 accessory armature. One specimen had two ac- 

 cessory reservoirs, one with five stylets and the 

 other, which appeared to be double, with five and 

 six stylets. Another specimen had three stylets in each of two reservoirs and yet another 

 had five reservoirs crowded together on one side of the main stylet each with four or 

 five. I could not determine the nerves in the proboscis. 



Anatomy. The head glands open at a median ventral pit. They are very large, filling 

 the head and persisting dorsally to the posterior end of the brain, ventrally farther back 

 still. 



The basement membrane is thick, nearly as thick as the epithelium. It does not stain. 

 The circular muscle layer is about half as thick as the basement membrane and the longi- 

 tudinal layer about as thick as the epithelium but thicker ventrally. 



The vascular system is well developed on the usual plan, but the vessels are much 



Fig. 46. Tetrastemma esbenseni, n.sp. Heads 

 of two specimens cleared in cedar oil showing 

 variation in eyespots. 



