Page:EB1911 - Volume 19.djvu/382

 This order represents the Schizonemertini of Hubrecht and the family Eupolidae.

The first three orders, which have a double muscular layer, external circular and internal longitudinal, are sometimes grouped together as the ; the Heteronemertini, in which a third coat of longitudinal muscles arises outside the circular layer, are then placed in a second branch, the.

The following families and genera are represented on the British coasts:, Carinella; , Cephalothrix, Carinoma;, Eunemertes; , Ototyphlonemertes;, Amphiporus, Drepanophorus;, Tetrastemma, Prosorhocmus; , Malacobdella;, Eupolia, Valencinia, Oxypolia;, Lineus, Euborlasia, Micrura, Cerebratulus, Micrella.

—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 into a single exterior opening), there is a terminal opening, the rhynchostome (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 as if it were itself a worm, a phenomenon which is at least partially explained by the extraordinary development of nervous tissue, equally distributed all through the walls of the proboscis, and either united into numerous longitudinal nerve-stems (Drepanophorus, Amphiporus) or spread out into a uniform and comparatively thick layer (Cerebratulus, sp.). This very effective and elaborate innervation, which has been directly traced to the brain, whence strong nerves (generally two) enter the proboscis, renders it exceedingly probable that the most important functions of the proboscis are of a sensiferous, tactile nature. 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 carried 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-evertible portion of the proboscis—a boundary which in the Metanemertini 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 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, or rhynchocoel, 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, the rhynchocoel. 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 contractions 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 enormously 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 rhynchocoel is formed by a split which appears in the mesoblast surrounding the epiblastic pit which is the forerunner of the proboscis. It does not seem to be coelomic.

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 short tube between this circular insertion and the rhynchostome is called the rhynchodaeum.

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. 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 purpose. The significance of two or more (in Drepanophorus very numerous) small sacs containing so-called “reserve” stylets resembling in shape that of the central dart is insufficiently known.

The muscular walls of the rhynchocoel, which by their transverse contractions serve to bring about eversion of the proboscis in the way above traced, are attached to the musculature of the head just in front of the ganglionic commissures (fig. 3). In nearly all Nemertines the rhynchocoel 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