170 TBTEASTBMMA FLAVIDA. 



Body. — Three fourths to one inch in length, and about a line in breadth, moderately elon- 

 gated, dilating very gradually behind the head, continuing for some distance of considerable 

 diameter, and again diminishing towards the tail. 



Colour. — Dull. whitish, salmon, or pinkish, with two elongated dark spots on the head. In 

 a specimen from St. Peter Port, Guernsey, the digestive tract was pale siskin-green. There was 

 also a faint white streak on the middle of the dorsum, commencing between the posterior pair 

 of eyes, and proceeding a short distance backwards. 



Bead wider than the succeeding portion of the body, flattened, having its greatest diameter 

 in the middle, and marked by the cephalic furrows. Eyes four, the pairs separated by a con- 

 siderable interval, which on each side is nearly filled up by a longitudinal patch of dark pigment. 

 The latter is widest anteriorly, and often does not quite reach the posterior eye, which is thus 

 prominent, while the anterior is indistinct. There is sometimes an opaque whitish patch between 

 the anterior pair of eyes, and this is continued faintly along the central streak. 



Cephalic furrows. — A little in front of the posterior pair of eyes is the groove connected 

 with the cephalic pit, which (furrow) passes inwards and slightly backwards on each side and 

 soon terminates. Somewhat behind the posterior pair of eyes another furrow slants inwards and 

 backwards, and meets its fellow of the opposite side in the middle line. 



T. vermicula in its lively and restless habits much resembles T. Candida. Many perish by 

 crawling out of the water and being dried on the side of the vessel. 



The ova are deposited in a free condition about the beginning of May. 



I have placed this pretty species under the name of M. de Quatrefages, but with amended 

 characters. He erroneously states that the head is not distinguished from the rest of the body, 

 and that the marginal stylet-sacs are situated on the " dorsal and ventral " aspects of the pro- 

 boscis. His figure, also, of the entire animal is too elongated, and his remark that the pigment- 

 patch between the eyes of each side is violet can only refer to the aspect under transmitted light. 

 Stimpson has a Tetrastemma (?) vermiculus in his 'Prodromus' (ii, p. 19), but its identity with 

 the present form is doubtful. 



5. Tetrastemma flavida, Bhrenberg. Plate IV, fig. 1. 



Specific character. — Head not wider than the rest of the body. Anterior and posterior pairs 

 of eyes widely separated. 



Synonyms. 



1831. Tetrastemma flavidum, Ehrenberg. Symb. Phys., Phyt. Turb., No. 25, tab. 5, f. 3, a— d, and 



a* — #; 

 1844-5. „ longecapitatum, (Ersted. Kroyer's Naturhist. Tids., i, p. 418. 



„ „ flavidum, Ibid. Op. cit., iv, p. 576, in note. 



Ibid. Entwurf Plattw., p. 87. 

 1846. Polia sangmrubra, De Quatrefages. Ann. des sc. nat., 3 me ser., Zool., torn, vi, p. 208, tab. 11, 



f. 3 and 7; tab. 12, f. 1. 

 1849. „ „ Ibid. Voyage en Sicilie, voUii, p. 120, pi. 15, f. 11 and 12. 



