84 



NEMERTINES 



P.O. 



brain-lobes; JV, lateral nerves; 

 PS, proboscidian sheatli ; /';, 

 proboscis; P.O., exterior open- 

 ing through which the probos- 

 cis is everted. (Esopliapus iiml 

 mouth shown by dotted lines. 



characters disclosed by the very important less highly 

 organized genera. 



A.na- ANATOMY. (a) Proboscis and Proboscidian Sheath. The 



tomy 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 opening), 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- 

 bratulus, 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, 



% ,. , l'io. 3. Anterior portion of the 



renders it exceedingly probable body of a Nemertine. 

 that the most important functions 

 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 

 ible portion of the pro- 

 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. 4. 



Fig. 5. 



5. Proboscis with stylet, ''reserve" 

 sacs, and muscular bulb of a Iioplonemer- 

 tine. i'ig. 4 retracted ; fig. 5 everted. 



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) FlG 6 _ Tlle ar . 

 where the stylet is attached serve for this pur- mature from 



m.' *e r / ">e proboscis 



pose. The significance of two or more (in O f Drepano- 

 Drepanophorus very numerous) small sacs con- P hor >">- 

 taining so-called " reserve " 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, 



