660 M O L L U S C A [PULMONATA. As in other Molluscan groups, we find even in closely- allied genera (for instance, in Aplysia and Pleurobran- chidium, and other genera observed by Lankester) the greatest differences as to the amount of food-material by which the egg-shell is encumbered. Some form their Diblastula by emboly (fig. 7), others by epiboly (fig. 5) ; and in the later history of the further development of the enclosed cells (arch-enteron) very marked variations occur in closely-allied forms, due to the influence of a greater or less abundance of food-material mixed with the protoplasm of the egg. Order 2 (of the Euthyneura). Pulmonata. Characters. Euthyneurous Anisopleurous Gastropoda, probably derived from anjc.stral forms similar to the Palliate Opisthobranchia by adaptation to a terrestrial life. The ctenidium is atrophied, and the edge of the mantle-skirt is fused to the dorsal integument by concrescence, except at one point which forms the aperture of the mantle-chamber, thus converted into a nearly closed sac. Air is admitted to this sac for respiratory and hydrostatic purposes, and it thus becomes a lung. An operculum is never present ; a contrast being thus afforded with the operculate Pulmonate Streptoneura (Cyclostoma, &c.), which differ in other essential features of structure from the Pulmonata, The Pulmonata are, like the other Euthyneura, hermaphrodite, with elaborately-developed copulatory organs and accessory glands. Like other Euthyneura, they have very numerous small denticles on the lingual ribbon. The ancestral Pulmonata appear to have retained both the right and the left osphradia (Spengel s olfactory organs), since in some (Planorbis, Auricularia) we find the single osphradium to be that of the original left side, whilst in others (Limnseus) it is that of the original right side. In some Pulmonata (Snails) the foot is extended at right angles to the visceral hump, which rises from it in the form of a coil as in Streptoneura ; in others the visceral hum}) is not elevated, but is extended with the foot, and the shell is small or absent (Slugs). The Pulmonata arc divided into two sub-orders according to the position of the cephalic eyes. Sub-order 1. Basommatophora. Characters. Eyes placed mediad of the cephalic tentacles at their base ; the embryonic velar area retained in adult life as a pair of cephalic lobes (fig. 70, v) ; male and female generative apertures separate, placed (as is typical in Anisopleura) on the right side of the neck ; visceral hump well developed, with a well-developed shell ; aquatic in habit. Family 1. Limnseldfe. Genera : Limnieus, Lam. (ligs. 3, 4, &c. ) ; Chilinia, Gray ; Physa, Draparn. ; Ancylus, Geoff. ; Planorbis, Mull., &c. Family 2. Auriculldse. Genera : Auricula, Lam. ; Conovulus, Lam. ; Pitharella, Wood. &c. Sub-order 2. Stylommatophora. Characters. Eyes placed on the summit of two hollow tentacles ; visceral hump well or not at all developed ; shell large and coiled, or minute or absent ; almost exclusively terrestrial. Family l.Helicitise. Genera: Helix, L. (figs. 69, A; 72*); Vitrina, Draparn. ; Siic- cinea, Draparn. ; Buliiims, Scopoli ; Achatina, Lam. ; Pupa, Lain. ; Clausilia, Draparn., &c. Family 2. I/imaridoB (Slugs). Genera : Limax, L. ; Incilaria, Benson ; Arion, Ferussac (fig. 69, D) ; Parmacella, Cuvier ; Tcstacclla, Cuvier (fig. 69, C), &c. Family 3. Oncidiadse. Genera : Oncidium, Buchanan ; Peronia, Blainv. (fig. 72) ; Faginulns, Ferussac, &c. Further He-marks on Pulmonata. The land-snails and slugs forming the group Pulmonata are widely distinguished from a small set of terrestrial Azygobranchia, the Pneumo- nochlamyda (see above), at one time associated with them on account of their mantle-chamber being converted, as in Pulmonata, into a lung, and the ctenidium or branchial plume aborted. The Pneumonochlamyda (represented in England by the common genus Cyclostoma) have a twisted FIG. 60. A series of Stylommatophorous Pulmonata, showing transitional forms between snail and slug. A. Helix pomatia (from Keferstein). B. Helicophanta brevipes (from Keferstein, after Pfeiffer). C. Testacella haliotidea (from Keferstein). D. Arion ater, the great Black Slug (from Keferstein). a, Shell in A, B, C, shell-sac (closed) in D ; b, orifice leading into the subpallial chamber (l ln K)- visceral nerve-loop, an operculum on the foot, a complex rhipidoglossate or taenioglossate radula, and are of distinct sexes ; they are, in fact, Azygobranchiate Streptoneura. The Pulmonata have a straight visceral nerve-loop, never an operculum (even in the embryo), and a multidenticulate FIG. 70. -A, B, C. Three views of Limnteits stagnalis, in order to show the persistence of the larval velar area v, as the circum-oral lobes of the adult. m, mouth ; /, foot ; v, velar area, the margin v corresponding with the ciliated band which demarcates the velar area or velum of the embryo Gas tropod (see fig. 4, D, K, F, II, I, () (Original.) radula, the teeth being equi-f ormal ; and they are hermaphro dite. Some Pulmonata (Limnseus, &c.) live in fresh-waters although breathing air. The remarkable discovery has been made that in deep lakes such Limnsei do not breathe air, but admit water to the lung-sac and live at the bottom. The lung-sac serves undoubtedly as a hydrostatic apparatus in the aquatic Pulmonata, as well as assisting respiration. It is not improbable that here, and in other air-breathing animals, the hydrostatic function was the primary one, and
the respiratory a later development.