Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/342

Rh 328 N E M E R T I N E S 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 Schizonemertines these elements are separated by a thin homogeneous base ment 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 Hoplonemertines 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). (c) 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. Never theless 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 longi tudinal, and an inner circular layer of muscular fibres circ.2. Fig. 7. Fig. 8. Fig. 9. FIGS. 7-9. The layers of the body-wall in Carinella (fig. 7), the Hoplonemertea (fig. 8), and the Schitonemertea (fig. 9). c, cellular tissue of the integument ; Jim, basement membrane ; Vc. 1, outer circular, and long, longitudinal layer of muscular tissue ; circ. 2, long. 1, additional circular and longitudinal layers of the same ; nl, nervous layer. (fig. 7). The second is common to all the Schizonemer tines as well as to Polia and Valencinia, and also compre hends three layers, of which, however, two are longitudinal, viz., the external and the internal one, there being a strong circular layer between tnem (fig. 9). To the third type all the Hoplonemertea correspond; their muscular layers are only two, an external circular and an internal longi tudinal one (fig. 8). The Schizonemertea thus appear to have developed an extra layer of longitudinal fibres internally to those which they inherited from more primitive ancestors, whereas the Hoplonemertea 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 Schizonemertea. In only one instance has the present writer met with a thin exterior circular layer in a very large specimen of Cerebratulus ; younger specimens of the same species did not show it. It is noticeable that Kefer- stein (9) also observed four layers similarly arranged in one of the specimens of Cerebratulus which he investi gated. 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. In Carinella, Cephalothrix, and Polia, as well as in all Hoplonemertines, the basement membrane of the skirt already above alluded to is particularly strong and immedi ately applied upon the muscular layers. In the Schizo- nemertines 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 base ment membrane sometimes wholly homogeneous, some times eminently fibrillar can more especially be observed in differently preserved specimens of the genus Polia. The connective tissue of the integument and basements 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 (Akrostomuni) 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 (&quot; reticulum,&quot; Barrois) which lies between the muscular body-wall and the intestine (1) is directly continuous with that in which the muscular layers are imbedded. Nuclei are everywhere present. The omni presence of this connective tissue excludes the idea of any true body cavity in Nemertines. In Polia the connective tissue enclosed in the external muscular layer is eminently vacuolar, all the interme diate stages between such cells in which the vacuole pre dominates 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 (4). The genital products develop in intermediate spaces similarly limited by these dissepiments and alternating with the digestive caeca. (d) 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 F u Scllizonemei. tin e (flg . io } and a Hoplo- brought about by a double Com- nemertine(fig.ll). eo, exterior opening; missure, of which the ventral .?., superior brain-lobe; p./., posterior portion is considerably thicker brain-lobe. than the dorsal, and which, together with the brain-lobes, consti tutes 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 ven tral commissure at the same time being extremely bulky. There is great probability that the central stems, together with the brain, eo p.1*. Fig. 10. Fig. 11. FIGS. 10, 11. Brain and lateral organ of