Page:EB1911 - Volume 19.djvu/385

 caeca are always eminently regular. So they are throughout the whole body in most of the Metanemertines. In Carinella they are generally deficient and the intestine straight; in young specimens of this species, however, they occur, though less regular and more in the form of incipient foldings by which the digestive surface is , increased. The inner surface of the intestinal caeca is ciliated, the caeca themselves are sometimes—especially in the hindermost portion of the body—of a considerably smaller lumen than the intermediate genital spaces; sometimes, however, the reverse is the case and in both cases it is the smaller lumen that appears enclosed between and suspended by the transverse fibres constituting the muscular dissepiments above mentioned.

The anus is situated terminally, the muscular body-wall through which the intestine must find its way outwards probably acting in this region the part of a sphincter. The lateral nerve stems mostly terminate on both sides in closest proximity to the anus; in certain species, however, they interfuse by a transverse connexion above the anus. The longitudinal blood-vessels do the same.

Circulatory apparatus.—The chief vessels are three longitudinal trunks, a median and two lateral ones. They are in direct connexion with each other both at the posterior and at the anterior end of the body. At the posterior end they communicate together by a T-shaped connexion in a simple and uniform way. Anteriorly there is a certain amount of difference in the arrangement. Whereas in the Metanemertines an arrangement prevails as represented in fig. 18, in the Heteronemertines the lateral stems, while entirely uniform all through the posterior portion of the body, no longer individually exist in the oesophageal region, but here dissolve themselves into a network of vascular spaces surrounding this portion of the digestive tract. The median dorsal vessel, however, remains distinct, but instead of continuing its course beneath the proboscidian sheath it is first enclosed by the ventral musculature of this organ, and still farther forwards it even bulges out longitudinally into the cavity of the sheath. Anteriorly it finally communicates with the lacunae just mentioned, which surround the oesophagus, bathe the posterior lobes of the brain, pass through the nerve ring together with the proboscidian sheath, and are generally continued in front of the brain as a lacunar space in the muscular tissue, one on each side.

Special mention must be made of the delicate transverse vessels regularly connecting the longitudinal and the lateral ones. They are metamerically placed, and belong to the same metamere as the digestive caeca, thus alternating with the generative sacs. The blood fluid does not flow in any definite direction; its movements are largely influenced by those of the muscular body-wall. It is colourless and contains definite corpuscles, which are round or elliptical, and in many Metanemertines are coloured red by haemoglobin, being colourless in other species. The circulatory system of Carinella is considerably different, being more lacunar and less restricted to definite vascular channels. Two lateral longitudinal

lacunae form, so to say, the forerunners of the lateral vessels. A median longitudinal vessel and transverse connecting trunks have not as yet been detected. There are large lacunae in the head in front of the ganglia.

The vascular system is entirely closed. It contains a colourless fluid, with flat, oval nucleated corpuscles, as a rule colourless, but in some cases tinged with yellow or red haemoglobin. Its presence is one of the most distinctive features which separate the Nemertines from the Platyhelminthes. In origin the vascular system is due to a fusion of spaces which arise in the mesoblast of the larva. The blood is probably circulated by the general contraction of the whole animal, since it is very doubtful if there are any intrinsic muscles in the vessel-walls. Its function is less that of respiration than of conveying the digested food-products all over the body, and the excretory products to the nephridia, and doubtless it serves at times to assist in the extension and retraction of parts of the body. The vessels in the more highly-developed genera seem to be partly lacunae and partly true vessels with definite walls.

Nephridia.—Associated with the lateral blood-vessels are the single pair of nephridia. Each consists of a more or less coiled, ciliated, longitudinal canal, which on its external surface gives origin to one or more transverse canals, which pass to the exterior and open a little way behind the mouth on the sides of the body. On its inner surface the longitudinal canal is adpressed to the lateral blood-vessel, and gives off a number of small, blind caeca or tags, each of which ends in a small clump of cells. These tags indent the blood-vessel. From their inner ends, projecting into the lumen of the tag, hangs a bunch of cilia, which forms the flickering “flame” so well known in the excretory apparatus of the Platyhelminthes and larval Annelids (fig. 19). There is no communication between the nephridia on one side and the other, but in Eupolia there are ducts opening into the alimentary canal as well as to the exterior, a condition of things which recalls what obtains in certain Oligochaetes. As a rule these organs only extend a short way along the anterior end of the body, a concentration which we may associate with the development of a vascular system to bring the products of excretion to a fixed spot. In Stichostemma, however, Montgomery has described a series of nephridia lying all along the body, and each with a varying number of external pores. The excretory system is epiblastic in its origin.

The two external openings of the nephridia are situated sometimes more towards the ventral, at other times more towards the dorsal side. Even in the larger Heteronemertines these pores are only a few millimetres behind the mouth region. In transverse sections the nephridia can be shown to be generally situated in the region limited by (1) the proboscidian sheath, (2) the upper wall of the intestine, (3) the muscular body-wall. No trace of nephridia is found posterior to the oesophagus.

Generative System.—In the Nemertines the sexes are separate, with only very few exceptions (Tetrastemma hermaphroditica, Marion). The reproductive system is of the simplest, strongly contrasting with the complicated arrangements in the Platyhelminthes. A series of sacs lined with an epithelium, the proliferation of which gives rise to the ova or spermatozoa, alternate between the caeca of the intestine. When mature, each sac pushes out a process to the exterior, and this forms the genital duct. The line of the genital openings is usually dorsal to the lateral nerve. The whole sac, with its epithelial wall and its contained genital cells, arises ultimately from some of the parenchymatous cells of the body. The walls and contents in some forms arise simultaneously; in others the walls are first formed and their lining then proliferates. It has been pointed out that the cavity of the sacs corresponds in many particulars with the coelom of higher animals, and in Lebidinsky’s observations on the development there is some support to the view that a coelom exists. Montgomery has also described certain spaces which may be coelomic lying between the alimentary canal and the inner longitudinal layer of muscles in the Heteronemertini. The ova and