210 CEPHALOTHRIX LINEARIS. 



Colour. — Variable. Sometimes the animal is of a pale cream-colour throughout, with no 

 special pigmentary accumulation. A patch of yellowish pigment occurs in other cases on the 

 snout, and the oesophageal region is yellowish ; or the yellowish, orange or reddish pigment is 

 increased towards the tip of the snout, and the oesophageal region is reddish-orange. The suc- 

 ceeding part is also faintly tinged in those most deeply coloured, the rest of the animal being of 

 the usual dull whitish or skin colour, and more opaque than the former. In a female specimen 

 laden with ova, sent from the St. Andrews rocks in April, the entire digestive cavity was of a 

 fine dark green hue (Plate IV, fig. 5), a condition probably due to the absorption of colouring 

 matter from the food, as specimens kept in vessels beside the ova of Phyllodoce maculata, Johnst., 

 become similarly tinged towards the posterior part of the digestive tract. 



Head. — Rounded, slightly tapered to a blunt point, not distinguished from the rest of the 

 body ; without eye-specks, and devoid of furrows or fissures. The mouth forms a conspicuous 

 slit a little behind the commencement of the oesophagus, and the pouting lips would seem 

 to be occasionally used as a kind of sucker, since a jerk occurs on raising the body from 

 this point. 



C. linearis is easily kept in confinement, moving about actively, or reclining at ease along 

 the vessel as a slender thread. It is fond of associating with fellows of the same or a similar 

 species, forming a tangled bunch or grouped as a radiating series of living filaments. In 

 progression the mobile snout is used as an exploratory organ, being thrust hither and thither with 

 ceaseless energy under a glass cover, and pushing aside its own yielding body in any 

 direction. The latter is also frequently drawn through a loop of mucus like a thread of coherent 

 yet fluid substance, which becomes thickened or attenuated by each successive contractile wave; 

 and it is sometimes bent in a peculiar manner from twists round loops of mucus or the bodies 

 of others. The skin is strongly acid to test-paper. 



The ova and spermatozoa are ripe from January to June. 



I have taken the Planaria linearis of Jens Rathke to be the present species. It was noticed 

 by Col. Montagu and afterwards by Dr. Johnston in Britain. The Lineus spiralis of the 

 former (MS.) is probably a variety to which the description exactly applies, with the exception of 

 the " red spiral intestine." The proboscis may occasionally be tinted. The presence of pigment- 

 or eye-specks does not seem to be of sufficient weight to separate (Ersted's C. bioculata and 

 C. cceca, if, indeed, the former is to be included in this genus at all. The Astemma rufifrons 

 of the same author is only a variety of the present species with a greater development of 

 pigment in the snout. The woodcut (op. cit., fi.g. 13), however, very much resembles a com- 

 pressed anterior region of Lineus bilineatus viewed as a transparent object. It is somewhat 

 doubtful if his Astemma longum (' Kroyer's Naturhist. Tidskrift') is a Cefthalothrix, though he 

 places it next A. rufifrons. Sir J. Dalyell was the first to notice the development of this species, 

 which he saw producing a rope of spawn in June. He correctly describes the young as having 

 two eyes. The Poliafilum of De Quatrefages has much resemblance to C. linearis; indeed, his 

 remarks can only apply to this form or to Nemertes carcinopMla, and the probability is in favour 

 of the former, though he had omitted to notice the mouth. The enlarged drawing of the 

 ganglia given by this author still further indicates the connection. This arrangement is 

 never seen in an example of the Enopla. The C. lineata ((Erst.) which M. Claparede found at 

 Skye is evidently this common species. There is also nothing in the descriptions and figures of 

 Prof. Keferstein's C. ocellata and C. longissima to distinguish them from each other or from 



