BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 389 



posite animal, each perfect individual consisting of numerous and apparently 

 still unformed or imperfectly formed individuals." Modern researches do not 

 support any of these suppositions. Of the other British zoologists who have 

 examined these animals, Dr Williams,* while admitting the digestive nature of 

 this chamber, misinterpreted its true relations. He considered the organ as a 

 closed sac filled with a milky fluid, and having many diverticula, into which the 

 nutritive matter passed by exudation from the proboscis. He appears thus to 

 have drawn up his description from an Ommatoplean, which possessed no large 

 slit leading into the chamber. He denied the existence of the proper anus. 

 While thus deviating from the true structure of the parts, he was correct at least 

 in viewing the chamber as digestive, and quite independent of the generative 

 system placed to its exterior. Sir J. Gr. Dalyell,! whose untiring scrutiny of the 

 habits of such animals is worthy of all praise, saw a Borlasian (his Gordius 

 gesserensis) feeding by the ventral slit, which he therefore correctly termed the 

 mouth. Dr Johnston, in his Catalogue, observes — " There is another and much 

 larger aperture in front, behind and underneath the head. Long mistaken for 

 the mouth, this has been usually described of late as genital, but the orifice is 

 doubtful." M. Van Beneden does not demonstrate that the so-called biliary 

 elements are simply constituents of the wall of the digestive cavity, and not 

 special caeca attached to the sides of the canal. In Cerebratulus tcenia (his 

 C. (Erstedii) he states that the digestive canal is divided into three compart- 

 ments — the first short, and corresponding to the oesophagus ; the second twice or 

 thrice the length of the former, and representing the stomach ; the third extend- 

 ing to the posterior extremity of the worm and constricted at regular intervals, and 

 corresponding to the intestine. I have not as yet noticed this in the British 

 examples, which agree with the typical Borlasian form in the structure of the 

 chamber, although the external aperture or mouth is somewhat smaller. Prof. 

 Keferstein'sJ description of the cavity as applied to Borlasia, though brief, is 

 good, and his criticism of Van Beneden's view, in regard to the " liver" in the 

 same group, fair. 



Nervous System. — The cerebral ganglia or central organs form two large and 

 conspicuous pale red masses situated a short distance behind the snout of the 

 worm (Plate X. fig. 1). They differ in shape, as seen under slight pressure, from 

 the same organs in Ommatoplea, each half being narrower and more elongated, 

 so as to cause the entire arrangement to have the appearance of a horse-shoe 

 magnet. In some specimens, instead of being more deeply tinted than the rest 

 of the cephalic tissues, they are paler, on account of the deep red coloration of the 

 latter; while, in others, they can scarcely be distinguished under the dense 

 blackish-green coating of cutaneous pigment. They are surrounded by the usual 



* 



Rept. Brit. Assoc. 1851. f Powers of the Creator, vol. ii. p. 73. 



\ Zeitsch. f. wiss. Zool. xii. p. 70. 



VOL. XXV. PART II. 5 G 



