TETRASTEMMA FLAVIDA. 171 



1850. Tetrastemma flavidum, Diesing. Syst. Helm., vol. i, p. 257. 



„ Nemertes haematodes, Ibid. Op. cit., p. 270. 

 1860. Folia obscura (partim), Van Beneden. Mem. Acad. Belg., torn, xxxii; Recher. sur les Turb. 



(sep. copy), p. 23, pi. 4, f. 10. 

 1862. Tetrastemma flavidum, Diesing. Revis. der Turb., p. 289. 



„ , y sanguirubrum, Ibid. Op. cit., p. 290. 



„ „ longecapitatum, Ibid. Op. cit., p. 293. 



1869. ,, varicolor, Mcintosh. Trans. Roy. Soc. Edinb., vol. xxv, pt. ii, p. 339 et seq. 



Habitat. — Under stones between tide-marks and in fissures of rocks ; less common than 

 T. Candida. From Scotland to the Red Sea. 



Body one to one inch and a half in length, attenuated, flattened, nearly of equal diameter 

 throughout, except where slightly tapered towards head and tail. 



Colour pinkish or pale peach, from the hue of the digestive tract. The snout is translucent, 

 with a slight opacity between and rather in front of the posterior pair of eyes, and a pale patch 

 from the ganglia behind them. The lateral margins are pale. 



Head rather indistinctly defined, bluntly rounded at the tip, from the centre of which the 

 usual pale streak proceeds. The eyes are equidistant in each pair, and the latter are separated 

 from each other by a much longer interval than exists in T. Candida and the others. The anterior 

 eyes have the larger masses of pigment. 



Cephalic furrows. — The openings of the cephalic sacs are placed nearly opposite the first 

 pair of eyes, so that the anterior furrows are carried far forward. They slant inwards and back- 

 wards just behind the eyes, while inferiorly they are nearly transverse. The posterior furrows 

 lie a little behind the last pair of eyes, and, proceeding inwards and backwards, meet in the centre 

 of the dorsum. They have a direction forward and inwards on the inferior surface, but less 

 obliquely than on the dorsum ; thus, while the dorsal meet towards the posterior part of the 

 ganglia, the ventral coalesce near the anterior border of the latter. The two sets of furrows 

 are indicated by lateral notches. 



This species is more sluggish than T. Candida, and much more delicate. It resembles 

 Nemertes carcinophila in the slow, gliding manner in which it moves about the vessel, a very 

 gentle undulatory motion of the head and body taking place. 



The ova are developed in May. 



Prof. Ehrenberg first gave a description and drawing of this species. The equidistant 

 eyes, with the pairs widely separated, and the cephalic furrows passing inwards nearly opposite 

 the first pair, are fairly represented. The Tetrastemma assimile of (Ersted, no doubt, has the 

 anterior and posterior pairs of eyes widely removed, but this is the only character which can be 

 identified with the present species. The Folia sanguirubra of De Quatrefages, again, appears to 

 be a variety with tinted nuclei to the proboscidian discs (a phenomenon probably due to refrac- 

 tion of the rays of light), and the Sicilian Folia baculus of the same author differs only in the 

 somewhat more attenuated condition of the snout. His Folia armata is also closely allied in 

 external characters, but the presence of four stylet-sacs, if not accidental, is a distinguishing 

 feature. Two of these marginal stylet-sacs, according to this author, occur a considerable way in 

 front of the central stylet, or at the anterior part of the elongated stylet-region, and two behind, 

 opposite the basal apparatus of the central organ. M. van Beneden seems to have included 



