NEMERTES GRACILIS. 177 



1850. Omatoplea balmea, Diesing. Syst. Helm., vol. i, p. 249. 



» » gracilis, Ibid. Op. cit., p. 250. 



1862 - » balmea, Ibid. Revision der Turbell., p. 255. 



» » gracilis, Ibid. Op. cit., p. 255. 



1865 - » » Johnston. Catalogue Brit. Mus., p. 22, pi. ii a, f. 1 and 1*. 



1869 - w « Mcintosh. Trans. Roy. Soc. Edinb., vol. xxv, pt. ii, p. 334 et seq. 



Habitat. — Frequent under stones between tide-marks, and especially abounding under 

 tangle-roots at low water. No example, however, was met with during an excursion to the 

 Channel Islands. 



Body.— Eight to twenty inches in length, and about a line in breadth, flattened in progression, 

 and thrown into ever-varying wrinkles, dilatations and contractions. The diameter is nearly 

 uniform for a considerable distance behind the head, and then the body gradually tapers towards 

 the tail. It is occasionally attenuated to a mere thread. 



Colour.— Dull greyish-green on the dorsum, deepest in front, and somewhat duskier in the 

 centre from the hue of the alimentary tract. Some incline to bluish-green anteriorly, others are 

 dull olive throughout. There are two reddish specks (due to the ganglia) behind the head. In 

 some specimens the dorsum for a short distance posterior to the latter is marked with minute pale 

 specks or with black pigment-grains, the latter occasionally continuing to the tip of the tail. 

 The under surface is greyish-white, dappled in the ripe females with dull whitish spots from the 

 ova. The reddish colorations from the ganglia are best observed on this surface, and there is 

 also a pale streak in the oesophageal region. 



Head. — Somewhat spathulate, flattened, rather blunt at the tip, and slightly dilated laterally, 

 so as to be wider than the rest of the body. Eyes arranged in two or three groups on each side, 

 the anterior cluster containing the larger number, and being situated just within the pale border 

 of the snout. The number of eyes in this series ranges from five to nine, but, as usual, 

 it is variable. The next group is placed distinctly posterior, nearer the middle line of the 

 head, and consists of three, four, or five eyes, which in some are transversely arranged. The 

 third cluster lies at the anterior border of the ganglion, or, occasionally, proceeds backwards 

 along the outer border of the latter. The first-mentioned groups are visible from the ventral 

 surface. 



Cephalic furrows. — There are no evident furrows on the dorsum of the head, and inferiorly 

 there is only the central slit of the mouth. 



This species lives a considerable time in confinement. Sometimes, when it has stretched 

 its snout beyond the water-line, while its tail reaches the bottom of the vessel, it assumes a very 

 regular spiral arrangement. If the snout be loosened from its attachment, the body is at 

 once gracefully shortened, like the stalk of a Vorticella or a coiled spring. It can also attenuate 

 itself to an extreme degree. It secretes a very tenacious, transparent mucus, especially when 

 irritated, and envelopes itself therewith, apparently for protection. The fine threads of this 

 secretion are so tough that they support the animal when lifted above the surface of the water. 

 The skin is acid to test-paper. 



Nemertes gracilis spawns in May. 



This animal was discovered by Dr. Johnston on the coast near Berwick, a region that 

 became so rich in novelties under his patient and searching scrutiny. It was re-described many 

 years later by M. de Quatrefages under the name Nemertes balmea, from Brehat. The French 



23 



