Ambrosius Arnold Willem Hubrecht and Arthur Everett Shipley

“ In addition to the nerves starting from the brain-lobes just now especially mentioned, there is a double apparatus which can hardly be treated of in conjunction with the sense organs, because its sensory functions have not been sufficiently made out, and which will therefore rather be considered along with the brain and central nervous system. This apparatus is usually known under the name of the lateral organs. ”

“ 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. ”

“ 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. ”

“ Digestive System.—The anterior opening, the mouth, is situated ventrally, close to the tip of the head and in front of the brain in the Metanemertini, somewhat more backward and behind the brain in the other Nemertines. In most Heteronemertines it is found to be an elongated slit with corrugated borders; in the Metanemertines it is smaller and rounded; in Malacobdella and Akrostomum it, moreover, serves for the extrusion of the proboscis, which emerges by a separate dorsal opening just inside the mouth. The oesophagus is the anterior portion of the digestive canal ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ I. Protonemertini, in which there are two layers of dermal muscles, external circular and internal longitudinal; the nervous system lies external to the circular muscles; the mouth lies behind the level of the brain; the proboscis has no stylet; there is no caecum to the intestine. ”

“ Our knowledge of the development of the most primitive forms is scanty. Both Hetero- and Metanemertini have been more exhaustively studied than the other two groups, the first, as was noticed above, being characterized by peculiar larval forms, the second developing without metamorphosis. ”

“ In all Heteronemertines there is on each side of the head a longitudinal slit of varying length but generally considerable depth, in the bottom of which the dark red brain is very plainly visible by transparency. These slits are continued into the ciliated duct, being at the same time themselves very strongly ciliated. In life they are commonly rhythmically opened and shut by a wavy movement. ”

“ 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. ”

“ 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. ”

“ With respect to the sense organs of the Nemertines, we find that eyes are of rather constant occurrence, although many Heteronemertines living in the mud appear to be blind. The more highly organized species have often very numerous eyes (Amphiporus, Drepanophorus) , which are provided with a spherical refracting anterior portion, with a cellular “vitreous body,” with a layer of delicate radially arranged rods, with an outer sheath of dark pigment, and with a separate nerve-twig each, springing from a common or double pair of branches which leave the brain as n. ”

“ Here the pits split into two, one part ending in a sac lined with sensory epithelium, and embedded in nervous tissue, the other projecting backwards as a long, glandular, blind canal. The exit of these organs takes many shapes, of value in systematic work. ”

“ There are no exterior appendages of any kind. The colours are often very bright and varied. Nemertines live in the sea, some being common amongst the corals and algae, others hiding in the muddy or sandy bottom, and secreting gelatinous tubes which ensheath the body along its whole length. ”

“ 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 ”