VASCULAB SYSTEM. 113 



distinctly above the latter. In his figure no separation is made, and the dilated organ is con- 

 founded with the posterior part of the superior lobe of each ganglion, the duct or ciliated canal 

 running beneath. The development of these sacs in the very young Nemertean inside the 

 JPylidium, as recently narrated by E. Metschnikoff, confirms all our views of their relations. 



10. Eye-specks. 



These are simply masses of black pigment, arranged on the sides of the snout with greater 

 or less regularity, and without any special optical structure. The textures of the head and nerve- 

 fibres themselves are so unfavourable for observation that I have had difficulty in making out 

 nerve-branches thereto. A more definite structure is observed in the Enopla, both as regards 

 nervous elements and complexity of organization. Some of the Anopla have no eyes (a remark, 

 however, which does not apply to Linens marinus), or have them only temporarily in their young 

 state, like the Tornaria-larva of Balanoglossus or developing oysters and Terebratulce, while all 

 the known Enopla possess them. It is a curious fact that in transverse sections of the snout 

 (such as Plate XVIII, fig. 7) considerable pigment-specks are seen towards the ventral surface. 



11. Vascular System. 



The circulation in Linens diverges considerably from that in the Enopla, the vessels differing 

 in definition, size, coiling, and contents. The main trunks, indeed, somewhat resemble long 

 cavities, with contractile walls, within which floats a transparent fluid with corpuscles. I have 

 termed this system the circulatory, but the current is driven by the contraction of the vessels now 

 backward, now forward, so that it is rather a kind of oscillation. 



There are three great longitudinal trunks — confining the description at present to the region 

 behind the oesophageal division of the digestive tract — a dorsal (p) and two lateral or ventral, r, r, 

 in the various transverse sections, and in Plate XIX, figs. 4 and 5. These three vessels in Linens 

 were first mentioned by Rathke. The dorsal is a large trunk situated immediately 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 vessels 

 are distended in L. gesserensis and L. sanguineus, they occupy almost the entire breadth of 

 the worm under gentle pressure. They are frequently dilated in various ways, sometimes 

 irregularly moniliform, crenate, or simply distended as long pale spaces. The three trunks 

 are intimately connected by an array of simple and rather large transverse anastomosing 

 branches {y, Plate XIX, fig. 4), some of which are forked. The transverse branches have 

 special contractile walls, and are not mere random channels, as may be seen in the longitudinal 

 sections of the worms (Plate XVIII, figs. 6, 4). They are subject to the various changes of form 

 noted in the larger trunks. The great longitudinal vessels are further connected at the tip of 

 the tail (Plate XIX, fig. 5). The dorsal generally contracts from behind forward, and drives the 

 corpuscular fluid, not only to the front, but also 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 



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