Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/341

Rh NEMERTINES 327 proboscis; P.O., exterior open ing through which the probos cis is everted. CEsophagus and mouth shown by dotted lines. characters disclosed by the very important less highly organized genera. ANATOMY. (a) Proboscis and Proboscidian Sheath. The organ most characteristic of a Nemertine is without doubt the proboscis. With very few exceptions (Malacobdella, Akrostomum, where it has fused with the mouth to a single exterior operfing), there is a terminal opening (subterminal in Valencinia] at the foremost tip of the body, out of which the proboscis is seen shooting backwards and forwards, sometimes with so much force that both its interior attachments are severed and it is entirely expelled from the body. It then often retains its vitality for a long time, apparently crawling about as if it were itself a worm, a phenomenon which is at least partially explained by the extra ordinary development of nervous tissue, equally distributed all through the walls of the proboscis, and either united (10) into nu merous longitudinal nerve-stems (Drepanophorus, Amphiporus) or spread out into a uniform and comparatively thick layer (Cere- Iratulus, sp.). This very effective and elaborate innervation, which has been directly traced (6) to the brain, whence strong nerves (gene rally two) enter the proboscis, 5 . / -i i ITT FIG. 3. Anterior portion of the renders it exceedingly probable body of a Nemertine. Br, iVmr flip rnnsr irrmnrranf fnnpfinni brain-lobes; N, lateral nerves; mat the most important i unctions ps&amp;lt; probos cldian shea th; /&amp;gt;/-, of the proboscis are of a sensi- ferous, tactile nature, a supposi tion which is again strengthened by the fact that amongst the Rhabdoccel Turbellarians an organ which may be called the forerunner of the Nemertean proboscis has been proved (3) to be the morphological equivalent of the foremost tip of the body, which, as an organ of delicate touch, has acquired the property of folding inwards. In Nemertines the everted proboscis is retracted in the same way as the tip of a glove finger would be if it were pulled backwards by a thread situated in the axis and attached to the tip. The comparison may be car ried still further. The central thread just alluded to is represented in the Nemertean proboscis by that portion which is never everted, and the tip of the glove by the boundary between the evertible and non-evert- FlGS 4 6 ._ Prol)OSCi ible portion Of the pro- Sllcs &amp;gt; ani? muscular bulb of a Hoplon T_ i i t i tine. Fig. 4 retracted; fig. 5 everted. boscis a boundary which in the Hoplonemertini is marked by the presence of a pointed or serrated stylet. This stylet is thus situated terminally when the proboscis has reached its maximum eversion. It adds a decisively aggressive character to an organ the original significance of which, as we have seen, was tactile. This aggressive character has a different aspect in several genera which are destitute of a central stylet, but in which the surface that is turned outwards upon eversion of the proboscis is largely provided with nematocysts, sending the urticating rods of different sizes in all directions. In others this surface is beset with thick, glandular, adhesive papillae. The comparison with the glove-finger is in so far Fig. 5. stylet, reserve emer- insufficient as the greater portion of the non-evertible half of the proboscis is also hollow and clothed by glandular walls. Only at the very hindermost end does it pass into the so-called retractor-muscle (fig. 2), which is attached to the wall of the space (proboscidian sheath) in which the proboscis moves about. This retractor-muscle, indeed, serves to pull back with great rapidity the extruded proboscis, and is aided in its action by the musculature of the head. The extrusion itself depends entirely upon contraction of the muscular walls of the space just mentioned (proboscidian sheath). As it is (1) closed on all sides, and (2) filled with a corpuscular fluid, the contractions alluded to send this fluid to impinge against the anterior portion, where the proboscis, floating in its sheath, is attached with it to the muscular tissue of the head (fig. 3). Partial extrusion lessening the resistance in this region inevitably follows, and when further con tractions of the walls of the sheath ensue total extrusion is the consequence. It is worthy of notice that in those Nemertines which make a very free use of their proboscis, and in which it is seen to be continually protruded and retracted, the walls of the proboscidian sheath are enor mously muscular. On the other hand, they are much less considerably or even insignificantly so in the genera that are known to make a rather sparing use of their proboscis. The proboscis, which is thus an eminently muscular organ, is composed of two or three, sometimes powerful, layers of muscles one of longitudinal and one or two of circular fibres. In the posterior retractor the longitudinal fibres become united into one bundle, which, as noticed above, is inserted in the wall of the sheath. At the circular insertion of the proboscis in front of the brain the muscular fibres belonging to the anterior extremity of the body and those connected with the proboscis are very intimately interwoven, forming a strong attachment. The proboscis broken off and expelled is generally reproduced, the posterior ribbon-like end of this reproduced portion again fusing with the walls of the sheath (11). There is reason to suppose that, when a wound is inflicted by the central stylet, it is envenomed by the fluid secreted in the posterior proboscidian region being at the same time expelled. A reservoir, a duct, and a muscular bulb in the region (fig. 4) where the stylet is attached serve for this pur pose. The significance of two or more (in Drepanophorus very numerous) small sacs con- P n &amp;gt; us - taining so-called &quot; reserve &quot; stylets resembling in shape that of the central dart is insufficiently known. The proboscidian sheath, which by its transverse con tractions serves to bring about eversion of the proboscis in the way above traced, and the muscular walls of which were similarly noticed, is attached to the musculature of the head just in front of the ganglionic commissures (fig. 3). In nearly all Nemertines it extends backwards as far as the posterior extremity, just above the anus; in Carinella it is limited to the anterior body-region. The corpuscles floating in the fluid it contains are of definite shape, and in Cerebratulus urticans they are deep red from the presence of haemoglobin. Internally the muscular layers are lined by an epithelium. In the posterior portion this epithelium in certain Schizonemertea 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 body-wall, 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. (b) Cutaneous System. Externally in all species a layer of ciliated cells forms the outer investment. In it are, FIG. 6. The ar mature from the proboscis of Drepano