BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 395 



Circulatory System. — The circulation in Borlasia diverges considerably from 

 that in Ommatoplea, the vessels differing in definition, size, coiling, and contents. 

 The main vessels indeed somewhat resemble long cavities, with contractile walls, 

 within which floats a transparent fluid with corpuscles. I have referred to this 

 system as the circulatory, but the current is driven by the contraction of the 

 vessels now backwards, now forwards, so that it is rather a kind of oscillation. 



There are three great longitudinal trunks— confining the description at pre- 

 sent to the region behind the oesophageal division of the digestive tract — a dorsal 

 (p) and two ventral, r, r in the various transverse sections, and in Plate XIII. fig. 2. 

 These three vessels in Borlasia were first mentioned by Rathke.* The dorsal is 

 a large trunk situated immediately to the outside and to the ventral surface of 

 the proboscidian sheath; while the ventral, also considerable trunks, lie on a 

 lower plane, and nearer the middle line than the nerves. Indeed, when the three 

 trunks are distended in B. olivacea and B. octocidata, they occupy nearly the 

 entire breadth of the worm under gentle pressure. These vessels are frequently 

 swollen in various ways, sometimes being irregularly moniliform from dilatations, 

 crenate, or simply distended as long pale spaces. The three trunks are inti- 

 mately connected by an array of simple and rather large transverse anastomos- 

 ing branches (y, Plate XIII. fig. 2), some of which are forked. These transverse 

 vessels have special contractile walls, and are not mere random channels, as may 

 be seen in the longitudinal sections of the worms (Plate XI. fig. 7, 4). They are 

 subject to the various changes of form noted in the larger trunks. The great 

 longitudinal trunks are further connected by meeting at the tip of the tail (Plate 

 XIII. fig. 2). The dorsal vessel generally contracts from behind forwards, and 

 this causes the corpuscular fluid, not only to rush to the front, but also to flow 

 through the transverse branches into the lateral trunks. The latter propel their 

 contents in both directions. 



At the posterior end of the oesophageal division of the alimentary canal the 

 three great vessels, for the most part, lose their individuality, and, so far as I have 

 observed, form an elaborate mesh work of vascular spaces (u, u, Plate X. fig. 1) 

 around this organ, again meeting in the lacunae (s, s) in front of the cavity, and 

 bathing the bulbs of the cephalic sacs which lie therein. These lacunae or chan- 

 nels pass forwards to unite at the ganglionic commissures, and the granules of 

 the contained fluid may be seen rushing forwards in the one and backwards in 

 the other. In addition to the smaller meshes surrounding the oesophageal region, 

 two larger spaces are seen on each side of the proboscidian sheath in transverse 

 section, which may be held as the continuations of the dorsal vessel. The reticula- 

 tions formed by this system are seen under favourable conditions in the living 

 animal (e.g., as represented in Plate X. fig. 1), as well as in numerous transverse 



* Neueste Schriften, &c. Danzig, 1842. 



