124 



MOLLUSCA 



The same general range of body-form is shown in Pul- 

 inonata 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 Limnseus (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 haemoglobin, 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 JE.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. Fir..72. Hermaphroditerepro 



The male duct v.d becomes fleshy 

 and muscular near its termination 

 at the genital pore, forming the 

 penis />. Attached to it is a diver- 

 ticulum fl., in which the sperma- 

 tozoa which have descended from 

 the ovo-testis are stored and mo- 

 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- 



l>pan 

 (Hell 



den Snail (Helix hortensis). 

 ovo-testis ; v.e, hermaphro 

 dite duct ; E.d. t albuminipar 

 ous gland ; u, uterine dilata 

 tion of the hermaphrodite 

 duct; d, 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- 

 ing into the female duct ; v.d, 

 male duct (vas deferens); p, 

 penis ; fl., flagellum. 



theca is simple. In other species of Helix a second duct 

 (as large in Helix asjwsa 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 ps, 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 (Limnaeus) 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 O3sophageal 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 " reversed " 

 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 "house," 

 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 

 "hybernaculmn," more or less calcareous in nature, which 

 is secreted by the foot. In Clausilia 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 " clausilium." 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 " shell-gland " discovered by him in other Mol- 

 lusca (Pisidium, Pleurobranchidium, Neritina, etc.). 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 



