Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/689

Rh PULMONATA.] MOLLUSCA 661 The same general range of body-form is shown in Pul- monata as in the Natant Azygobranchia and in the Opis- thobranchia ; at one extreme we have Snails with coiled visceral hump, at the other cylindrical or flattened Slugs (see fig. 69). Limpet-like forms are also found (fig. 71, Ancylus). The foot is al ways simple, with its flat crawling surface extending from end to end, but in the embryo Limnaeus (fig. 4, H) it shows a bilobed character, which leads on to the condition characteristic of Pteropoda. The adaptation of the Pulmonata to ter- restrial life has entailed little modification of the internal organization. The vascular system appears to be more complete in them than in other Gastropoda, fine vessels and even capillaries being present in place of lacunae, in which arteries and veins find their meeting- point. The subject has not, however, been investigated by the proper methods of recent histology, and our know- form aquatic Pui- FIG. 72. Peronia Tonga;, a littoral Pulmonate, found on the shores of the Indian and Pacific Oceans (Mauritius, Japan). ledge of it, as of the vascular system of Molluscs generally, is most unsatisfactory. In one genus (Planorbis) the plasma of the blood is coloured red by hemoglobin, this being the only instance of the pre sence of this body in the blood of Glossophorous Mollusca, though it occurs in corpuscles in the blood of the bivalves Area and Solen (Lankester, 31). The generative apparatus of the Snail (Helix) may serve as an ex ample of the hermaphrodite appa ratus common to the Pulmonata and Opisthobranchia (fig. 72*). From the ovo-testis, which lies near the apex of the visceral coil, a common hermaphrodite duct v.e proceeds, which receives the duct of the compact white albumini- parous gland E.d., and then be comes much enlarged, the addi tional width being due to the development of glandular folds, which are regarded as forming a uterus u. Where these folds cease the common duct splits into two portions, a male and a female. Fic,.72*. Hermaphrodite repro ductive apparatus of the Gar den Snail (Helix hortensis). z, ovo-testis ; v.e, hermaphro dite duct ; E.d., albuminipar- ous gland ; it, uterine dilata tion of the hermaphrodite duct ; rf, digitate accessory glands on the female duct ; p.s, calciferous gland or dart- sac on the female duct ; R.f, spermatheca or receptacle of the sperm in copulation, open- delled into sperm ropes .or sperma- tophores. The female portion of the duct is more complex. Soon after quitting the uterus it is joined by a long duct leading from a glandular sac, the spermatheca (R.f}. In this duct and sac the spermatophores received in copulation from another snail are lodged. In Helix hortensis the sperma- The male duct v.d becomes fleshy and muscular near its termination at the genital pore, forming the penis p. Attached to it is a diver- ticulum /., in which the sperma tozoa which have descended from the ovo-testis are stored and mo- ing into the female duct ; v.d, male duct (vas deferens) ; p, penis ; fl. t flagellum. theca is simple. In other species of Helix a second duct (as large in Helix axpersa as the chief one) is given off from the spermathecal duct, and in the natural state is closely adherent to the wall of the uterus. This second duct has normally no spermathecal gland at its termination, which is simple and blunt. But in rare cases in Helix aspersa a second spermatheca is found at the end of this second duct. Tracing the widening female duct onwards we now come to the openings of the digitate accessory glands d, d, which probably assist in the formation of the egg-capsule. Close to them is the remarkable dart-sac ;, a thick-walled sac, in the lumen of which a crystalline four-fluted rod or dart consisting of carbonate of lime is found. It is supposed to act in some way as a stimulant in copulation, but pos sibly has to do with the calcareous covering of the egg- capsule. Other Pulmonata exhibit variations of secondary importance in the details of this hermaphrodite apparatus. The nervous system of Helix is not favourable as an example on account of the fusion of the ganglia to form an almost uniform ring of nervous matter around the oesophagus. The Pond-Snail (Limnreus) furnishes, on the other hand, a very beautiful case of distinct ganglia and connecting cords (fig. 22). The demonstration which it affords of the extreme shortening of the Euthyneurous vis ceral nerve-loop is most instructive and valuable for com parison with and explanation of the condition of the nervous centres in Cephalopoda, as also of some Opisthobranchia. The figure (fig. 22) is sufficiently described in the letter press attached to it ; the pair of buccal ganglia joined by the connectives to the cerebrals are, as in most of our figures, omitted. Here we need only further draw attention to the osphradium, discovered by Lacaze Duthiers (32), and shown by Spengel to agree in its innervation with that organ in all other Gastropoda. On account of the shortness of the visceral loop and the proximity of the right visceral ganglion to the cesophageal nerve-ring, the nerve to the osphradium and olfactory ganglion is very long. The posi tion of the osphradium corresponds more or less closely with that of the vanished right ctenidium, with which it is normally associated. In Helix and Limax the osphradium has not been described, and possibly its discovery might clear up the doubts which have been raised as to the nature of the mantle-chamber of those genera. In Planorbis, which is dexiotropic (as are a few other genera or exceptional varieties of various Anisopleurous Gastropods) instead of being leiotropic, the osphradium is on the left side, and receives its nerve from the left visceral ganglion, the whole series of unilateral organs being reversed. This is, as might be expected, what is found to be the case in all &quot; reversed &quot; Gastropods. It is also the case in the Pulmonate Auricula, which is leiotropic. The shell of the Pulmonata, though always light and delicate, is in many cases a well-developed spiral &quot; house,&quot; into which the creature can withdraw itself ; and, although the foot possesses no operculum, yet in Helix the aperture of the shell is closed in the winter by a complete lid, the &quot;hybernaculum,&quot; more or less calcareous in nature, which is secreted by the foot. In Clau.silia a peculiar modifica tion of this lid exists permanently in the adult, attached by an elastic stalk to the mouth of the shell, and known as the &quot; clausilium.&quot; In Limnseus the permanent shell is preceded in the embryo by a well-marked shell-gland or primitive shell-sac (fig. 72***), at one time supposed to be the developing anus, but shown by Lankester to be identical with the &quot; shell-gland &quot; discovered by him in other Mol lusca (Pisidium, Pleurobranchidium, Neritina, &c.). As in other Gastropoda Anisopleura, this shell-sac may abnorm ally develop a plug of chitonous matter, but normally it flattens out and disappears, whilst the cap-like rudiment of the permanent shell is shed out from the dome-like surface