NEMERTINES 



85 



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 Cnrinella, 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 (fiX's. 

 7,8). 



(r) 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, 

 Carindla and Cephalothrix, i.e., an outer circular, a longi- 

 tudinal, and an inner circular layer of muscular fibres 



Fig. 7. Fig. 8. Fig. 9. 



FIGS. 7-9. The layers of tlie body-wall in Carinella (fig. 7), the Iloplonemertea 

 (tic. 8), anil the Schizonemertea (fig. 9). c, cellular tissue of the integument ; 

 /lut, basement membrane ; cire. 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 tnern (fig. 9). To the third type 

 all the Ifaplonemertea 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 Gerebratulus ; 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 C<'r<l>i;iliiliix 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 C'ariiifl/ii, Cephalothrix, and 1'ulin, as well as in all 

 Hoplonemertines, the basement membrane of the skin 

 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 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," Barrels) 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 Nemertiues. 



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 resophagus, 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 

 Nemertiues presents several interesting peculiarities. As central 

 organs we have to note the brain-lobes anil 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 

 ords, and at the same time as 



the spot where the two halves 

 of the central nerve system 



_. J* . T."ii 



Fig. 10. 



Fig. 11. 



iiitmvnmmiirnVatp Ttii* is FlGS - 10 ' H. Brain and lateral organ of 



1JUS IS H schizoncmertine (tig. 10) and a Hoplo- 



brought about by a double com- nemerUne(flg.ll). eo, exterior opening; 

 missure, of which the ventral ".'., superior brain-lobe; p.l., 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 C'arinella 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, 



