MOLLUSCA 



115 



the mantle-skirt cut along its 

 left line of attachment and 

 thrown over to the right side 

 of the animal so as to expose 

 the organs on its inner face. 

 a, anus ; vg^ vagina ; gp, adrec- 

 tal purpuriparous giknA.: /, 

 aperture of the nephridimn (k id- 

 ney) ; 6r, ctenidium (branchial 

 plume); &/, paral)ranchia(=the 

 comb-like osptaradium or olfac- 

 tory organ). 



piuric- acid (as much as two per cent is present in the 

 secretion), which assists the animal in boring holes by 

 means of its rasping tongue through the shells of other 

 Molluscs upon which it preys. A crop-like dilatation of 

 the gut and a recurved intestine, embedded in the com- 

 pact yellowish-brown liver, the ducts of which open into it, 

 form the rest of the digestive tract and occupy a large 

 bulk of the visceral hump. The buccal region presents a 

 pair of shelly jaws placed laterally upon the lips, and a 

 wide range of variation in the form of the denticles of the 

 lingual ribbon or radula, the nature of which will be un- 

 derstood by a reference to fig. 9, whilst the systematic list 

 of families given above shows the particular form of den- 

 tition characteristic of each division of the order. 



The modification in the form of the snout upon which 

 the mouth is placed, leading to the 

 distinction of " proboscidif erous " 

 and " rostrif erous " Gastropods, re- , , v,^, __^ 



quires further notice. The condi- \^~^**=^ ^\^ '"^^'^ ' 

 tion usually spoken of as a " pro- 

 boscis " appears to be derived from 

 the condition of a simple rostrum 

 (having the mouth at its extrem- 

 ity) by the process of incomplete 



introversion, of that simple rostrum. '"'^ i c-vV ^r' 

 There is no reason in the actual „ „ „ 



. « c i_-\. T 1 ii Fig. 47. — Female of Purpwra la- 



signmcance or the word why the ptiius removed from its shell 

 term "proboscis" should be applied 

 to an alternately introversible and 

 eversible tube connected with an 

 animal's body, and yet such is a 

 very customary use of the term. 

 The introvBrsible tube may be 

 completely closed, as in the " pro- 

 boscis" of Nemertean worms, or 

 it may have a passage in it leading into a non-eversible 

 oesophagus, as in the present case, and in the case of the 

 eversible phar5rQx of the predatory Chsetopod worms. The 

 diagrams here introduced (fig. 48) are intended to show 

 certain important distinctions which obtain amongst the 

 various "introverts," or intro- and e-versible tubes so fre- 

 quently met with in animal bodies. Supposing the tube 

 to be completely introverted and to commence its ever- 

 sion, we then find that eversion may take place, either 

 by a forward movement of the side of the tube near its 

 attached base, as in the proboscis of the Nemertine worms, 

 the pharynx of Chsetopods, and the eye-tentacle of Gastro- 

 pods, or, by a forward movement of the inverted apex 

 of the tube, as in the proboscis of the Rhabdocoel Planar- 

 ians, and in that of Gastropods here under consideration. 

 The former case we call " pleurecbolic " (fig. 48, A, B, C, 

 H, I, K), the latter " acrecbolic " tubes or introverts (fig. 

 48, D, E, F, G). It is clear that, if we start from the 

 condition of fuU eversion of the tube and watch the pro- 

 cess of introversion, we shall find that the pleurecbolic 

 variety is introverted by the apex of the tube sinking in- 

 wards ; it may be called acrembolic, whilst conversely the 

 acrecbolic. tubes are pleurembolic. Further, it is obvious 

 enough that the process either of introversion or of eversion 

 of the tube may be arrested at any point, by the develop- 

 ment of fibres connecting the wall of the introverted tube 

 with the wall of the body, or with an axial structure such 

 as the oesophagus ; on the other hand, the range of move- 

 ment of the tubular introvert may be unlimited or complete. 

 The acrembolic proboscis or frontal introvert of the Nemer- 

 tine worms has a complete range. "So has the acrembolic 

 pharynx of Chmtopods, if we consider the organ as ter- 

 minating at that point where the jaws are placed and the 

 oesophagus commences. So too the acrembolic eye-tentacle 

 of the snail has a complete range of movement, and also the 



pleurembolic proboscis of the Rhabdocoel prostoma. The 

 introverted rostrum of the Azygobranch Gastropods pre- 

 sents in contrast to these a limited range of movement. 

 The " introvert " in these Gastropods is not the pharynx as 

 in the Chsetopod worms, but a prse-oral structure, its apical 

 limit being formed by the true lips and jaws, whilst the 

 apical limit of the Chaetopod's introvert is formed by the 

 jaws placed at the junction of pharynx and oesophagus, so 

 that the Chaetopod's introvert is part of the stomodseum 

 or fore-gut, whilst that of the Gastropod is external to the 

 alimentary canal altogether, being in front of the mouth, 

 not behind it, as is the Chsetopod's. Further, the Gastro- 

 pod's introvert is pleurembolic (and therefore acrecbolic), 

 and is limited both in eversion and in introversion ; it can- 



FiG. 48. — Diagi'ams explanatory of the natui'e of so-called proboscides or "intro- 

 verts." A. Simple introvert completely introverted. B. The same, partially 

 everted by eversion of the sides, as in the Nemertine proboscis and Gastropod 

 eye-tentacle = pleurecbolic. C. The same, fully everted. D, E. A similar 

 simple introvert in course of eversion by the forward movement, not of its 

 sides, but of its apex, as in the proboscidean Rhabdocoels= acrecbolic. F. 

 Acrecbolic ( = pleurembolic) introvert, formed by the snout of the proboscidi- 

 ferous Gastropod, al, alimentary canal ; d, the ti-ue mouth. The introvert 

 is not a simple one with complete range both in eversion and introversion, 

 but is arrested in introversion by the fibrous bands at c, and similarly in 

 eversion by the fibrous bands at &. G. The acrecbolic snout of a probos- 

 cidiferous Gastropod, arrested short of complete eversion by the fibrous baud 

 6. H. The acrembolic (= pleurecbolic) pharynx of a Chsetopod fully intro- 

 ■ verted, ai, alimentary canal ; at d^ the jaws ; at a, the mouth ; therefore a 

 to d is stomodseum, whereas in the Gastropod (F)ato d is inverted body- 

 surface. I. Partial eversion of H. K. Complete eversion of H. (Original.) 



not be completely everted owing to the muscular bands 

 (fig. 48, G), nor can it be fully introverted owing to the bands 

 (fig. 48, F) which tie the axial pharynx to the adjacent 

 wall of the apical part of the introvert. As in all such 

 intro- and e-versible organs, eversion of the Gastropod 

 proboscis is efiected by pressure communicated by the 

 muscular body-wall to the liquid contents (blood) of the 

 body-space, accompanied by the relaxation of the muscles 

 which directly pull upon either the sides or the apex of 

 the tubular organ. The inversion of the proboscis is effected 

 directly by the contraction of these muscles. In various 

 members of the Azygobranchia the mouth-bearing cylinder 

 is introversible (i.e., is a proboscis) — with rare exceptions 

 these forms have a siphonate mantle-skirt. On the other 

 hand, many which have a siphonate mantle-skirt are not 

 provided with an introversible mouth-bearing cylinder, but 

 have a simple non-introversible rostrum, as it ha.s been 



