G52 MOLLUSCA [AZYGOBRANCHIA. r; 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 gland ; r , aperture of the nephridium (kid ney) ; br, ctenidium (branchial plume); br , parabranchia( = the comb-like osphradium or olfac tory organ). phuric 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 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. There is no reason in the actual .., , , , , , Fir,. 47. Female of Pvrpura la- Slgnificance Of the Word Why the pillus 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 introversible 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 pharynx of the predatory Chtetopod 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 Chaetopods, 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 Rhabdoccel 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 full 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 Chaetopods, 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 Rhabdoccel 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 Chaetopod worms, but a prae-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 stomodieum 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 Chtetopod 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. Diagrams explanatory of the nature 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 it.s sides, but of its apex, as in the proboscidean Rhabdoccels = acrecl>olic. F. Acrecbolic ( = pleurembolic) introvert, fonned by the snout of the proboscidi- ferous Gastropod, al, alimentary canal ; rf, the true 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 6. G. The acrecbolic snout of a probos- cidiferous Gastropod, arrested short of complete eversion by the fibrous band b. H. The acrembolic ( = pleurecbolic) pharynx of a Chsetopod fully intro verted, nl, alimentary canal ; at rf, the, jaws ; at a, the mouth ; therefore a to d is stomodsnun, whereas in the Gastropod (F) a to 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 G astro] >od proboscis is effected 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 jyrobosci-s) 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-introversiblo rostrum, as it has been