Page:EB1911 - Volume 19.djvu/383

 shape, and in Cerebratulus urticans they are deep red, possibly from the presence of haemoglobin. They are usually larger than the blood corpuscles. Internally the muscular layers are lined by an epithelium. In the posterior portion this epithelium in certain Heteronemertea has a more glandular appearance, and sometimes the interior cavity is obliterated by cell-proliferation in this region. Superiorly the sheath either closely adheres to the muscular bodywall, with which it may even be partly interwoven, or it hangs freely in the connective tissue which fills the space between the intestine and the muscular body-wall.

Cutaneous System.—Externally in all species a layer of ciliated cells forms the outer investment. In it are, moreover, enclosed unicellular glands pouring their highly refracting contents, of a more or less rod-like shape, directly to the exterior. They appear to be the principal source of the mucus these animals secrete. In most Heteronemertines these elements are separated by a thin homogeneous basement membrane (fig. 8) from the following—that is, from a layer in which longitudinal muscular fibres are largely intermixed with tortuous glands, which by reason of their deeper situation communicate with the exterior by a much longer and generally very narrow duct. The pigment is also principally localized in this layer, although sometimes it is present even deeper down within the musculature. The passage from this tegumentary layer to the subjacent longitudinal muscular one is gradual, no membrane separating them. In Carinella, Cephalothrix, Polia and the Metanemertines the two tegumentary layers with their different glandular elements are fused into one; a thick layer of connective tissue is situated beneath them (instead of between them) and keeps the entire cutaneous system more definitely separate from the muscular (figs. 7, 8).

Musculature and Connective Tissue.—The muscular layers by which the body-wall is constituted have been very differently and to some extent confusingly described by the successive authors on Nemertean anatomy. There is sufficient reason for this confusion. The fact is that not only have the larger subdivisions a different arrangement and even number of the muscular layers, but even within the same genus, nay, in the same species, well-marked differences occur. Increase in size appears sometimes to be accompanied by the development of a new layer of fibres, whereas a difference in the method of preparation may give to a layer which appeared homogeneous in one specimen a decidedly fibrous aspect in another. Nevertheless there are three principal types under which the different modifications can be arranged. One of them is found in the two most primitively organized genera, Carinella and Cephalothrix, i.e. an outer circular, a longitudinal and an inner circular layer of muscular fibres (fig. 7). The second is common to all the Heteronemertines, as well as to Polia and Valencinia, and also comprehends three layers, of which, however, two are longitudinal, viz. the external and the internal one, there being a strong circular layer between them (fig. 9). To the third type all the Metanemertini correspond; their muscular layers are only two, an external circular and an internal longitudinal one (fig. 8).

The Heteronemertini thus appear to have developed an extra layer of longitudinal fibres internally to those which they inherited from more primitive ancestors, whereas the Metanemertini are no longer in possession of the internal circular layer, but have on the contrary largely developed the external circular one, which has dwindled away in the Heteronemertini. The situation of the lateral nerve-stems in the different genera with respect to the muscular layers lends definite support to the interpretation of their homologies here given and forms the basis of Bürger's classification.

In Carinella, Cephalothrix and Polia, as well as in all Metanemertines, the basement membrane of the skin already alluded to is particularly strong and immediately applied upon the muscular layers. In the Heteronemertines there is a layer in which the cutaneous elements are largely represented below the thin basement membrane (fig. 8), between it and the bulk of the outer longitudinal muscles. The difference in the appearance of the basement membrane—sometimes wholly homogeneous, sometimes eminently fibrillar—can more especially be observed in differently preserved specimens of the genus Polia.

The connective tissue of the integument and basement membrane imperceptibly merges into that which surrounds the muscular bundles as they are united into denser and definite layers, and this is especially marked in those forms (Akrostomum) where the density of the muscular body-wall has considerably diminished, and the connective tissue has thus become much more prominent. It can then at the same time be observed, too, that the compact mass of connective tissue (“reticulum,” Barrois) which lies between the muscular body-wall and the intestine is directly continuous with that in which the muscular layers are embedded. Nuclei are everywhere present. The omnipresence of this connective tissue tends to exclude the formation of any perivisceral body cavity in Nemertines.

In Polia the connective tissue enclosed in the external muscular layer is eminently vacuolar—all the intermediate stages between such cells in which the vacuole predominates and the nucleus is peripheral and those in which the granular protoplasm still entirely fills them being moreover present.

In addition to the musculature of the proboscis and proboscidian sheath, longitudinal muscular fibres are found in the walls of the oesophagus, whilst transverse ones are numerous and united into vertical dissepiments between the successive intestinal caeca, thus bringing about a very regular internal metamerization. The genital products develop in intermediate spaces similarly limited by these dissepiments and alternating with the digestive caeca.

Nervous System and Sense Organs.—The nervous system of Nemertines presents several interesting peculiarities. As central organs we have to note the brain-lobes and the longitudinal lateral cords which form one continuous unsegmented mass of fibrous and cellular nerve-tissue. The fibrous nerve-tissue is more dense in the higher differentiated, more loose and spongy in the lower organized forms; the cellular nerve-tissue is similarly less compact in the forms that are at the base of the scale. No ganglionic swellings whatever occur in the course of the longitudinal cords. The brain must be looked upon as the anterior thickening of these cords, and at the same time as the spot where the two halves of the central nerve system intercommunicate. This is brought about by a double commissure, of which the ventral portion is considerably thicker than the dorsal, and which, together with the brain-lobes, constitutes a ring through which both proboscis and proboscidian sheath pass. The brain-lobes are generally four in number, a ventral and a dorsal pair, respectively united together by the above-mentioned commissures, and moreover anteriorly interfusing with each other, right and left. In Carinella this separation into lobes of the anterior thickenings of the cords has not yet commenced, the ventral commissure at the same time being extremely bulky. There is great probability that the central stems, together with the brain, must be looked upon as local longitudinal accumulations of nervous tissue in what was in more primitive ancestors a less highly differentiated nervous plexus, situated in the body-wall in a similar way to that which still is found in the less highly organized coelenterates. Such a nervous plexus indeed occurs in the body-wall of all Heteronemertines, sometimes even as a comparatively thick layer, situated, as are the nerve stems, between the external longitudinal and the circular muscles (fig. 9). In Carinella, where the longitudinal nerve-stems are situated exteriorly to the muscular layers, this plexus, although present, is much less dense, and can more fitly be compared to a network with wide meshes. In both cases it can be shown to be in immediate continuity with the coating of nerve-cells forming part of the longitudinal cords. It stretches forward as far as the brain, and in Carinella is again continued in front of it, whereas in the Heteronemertines the innervation of the anterior extremity of the head, in front of the brain, takes the form of more definite and less numerous branching stems. The presence of this plexus in connexion with the central stems, sending out nervous filaments amongst the muscles, explains the absence, in Pro-, Meso- and Heteronemertines, of separate and distinct peripheral nerve stems springing from the central stems innervating the different organs and body-regions, the only exceptions being the