AZYGOBRANCHIA.] MOLLUSCA 651 the renal organ, and consists of a single auricle receiving Mood from the gill, and of a single ventricle which pumps it through the body by an anterior and posterior aorta (see fig. 105). The surface x of the mantle between the rectum and the gill- plume is thrown into folds which in many sea-snails (Whelks, & c .) are very strongly deve- l J rnu 1 1 Fio. 41. Animal and shell of Ovulum. h, cephalic lOpea. 1 lie WilOle tentacles ; d, foot ; h, mantle-skirt, which is natu- Of this Surface ap- rall y carried in a reflected condition so as to . L cover m the sides of the shell. pears to be active in the secretion of a mucous-like substance. The single gill-plume br lies to the left of the median line in natural position. It corresponds to the right of the two primitive cten- idia in the untwisted archaic condition of the Molluscan body, and does not project freely into the branchial cavity, but its axis is attached (by concres cence) to the mantle-skirt (roof of the branchial chamber). It is rare for the gill-plume of an Anisopleurous Gastropod to stand out freely as a plume, but occasionally this more ar chaic condition is exhibited, as in Valvata (fig. 45). Next be yond (to the left of) the gill- plume we find the so-called para branchia, which is here simple, but sometimes lamellated as in Purpura (fig. 47). This organ has, without reason, been sup- posed to represent the second FIQ 42 ^ Section of the 8hell of ctemdium of the typical Mollusc, which it cannot do on account of its position. It should be to the right of the anus were this the case. Recently Spengel has shown that the parabran- chia of Gastropods is the typical olfactory organ or osphradium in a highly-developed condition The minute structure of the epithelium which clothes it, as well as the origin of Tritonium, Cuv. a, apex ; oc, si- phonal notch of the mouth of the shell ; ac to pc, mouth of the shell; w, w, whorls of the shell ; s, s, su tures. Occupying the axis, and exposed by the section, is seen the " columella " or spiral pillar. The upper whorls of the shell are seen to be divided into separate cham bers by the formation of succes sively formed "septa." (From Owen.) Fro. 43. Animal and shell of Rostdlaria rectirostris. a, snout or rostrum ; 6, cephalic tentacle ; c, eye ; d, propodium and mesopodinm ; e, metapodium ; /, operculum ; h , prolonged siphonal notch of the shell occupied by the siphon, or trough-like process of the mantle-skirt. (From Owen.) the nerve which is distributed to the parabranchia, proves it to be the same organ which is found universally in Mol luscs at the base of each gill-plume, and tests the indrawn current of water by the sense of smell. The nerve to this roof of the branchial chamber la the notable character of this g nus - Fio. 44. Female Janthina, with egg-float (a) attached to the foot; b, egg- capsules; c, ctenidium (gill-plume); d, cephalic tentacles. organ is given off from the superior (original right, sec fig. 19) visceral ganglion. The figures which are here given of various Azygo- branchia are in most cases suffi ciently explained by the refer ences attached to them. As an excellent general type of the nervous system, attention may be directed to that of Paludina drawn in fig. 21. On the whole, the ganglia are strongly indivi dualized in the Azygobranchia, nerve-cell tissue being concen trated in the ganglia and absent Flo . ^_ Valmta ~ istafcti Mull . from the cords (contrast with Zv- > mouth ; P< operculum ; , ,. j T i A ctenidium (branchial plume) gObrancllia and ISOpleura). At filiform appendage (? rudim the same time, the junction of ?T cteniiHum> The freely J lecting ctenidium of typical form the Visceral loop above the 111- ""t having its axis fused to the 4-nc,4-^^ r^^r, 4^ n cu testme prevents in all StreptO- neura the shortening of the vis- ceral loop, and it is rare to find a fusion of the visceral ganglia with either pleural, pedal, or cerebral a fusion which can and does take place where the visceral loop is not above but below the intestine, e.g., in the Euthy neura (fig. 67), Cephalopoda(fig. 112), and Lamellibranchia (fig. 144). As con- trasted with the Zygo- branchia and the Iso- pleura, we find that in the Azygobranchia the pedal nerves are dis tinctly nerves given off from the pedal ganglia, rather than cord-like nerve - tracts contain ing both nerve -cells or ganglionic elements and nerve-fibres. Yet in some Azygobran chia (Paludina) a lad- Fio. 46. Male of lAttorina littoral!*, Lin., rp- . ,:, moved from its shell; the mantle-skir der-like arrangement of the two pedal nerves and their lateral branches has been de tected (30). The his tology of the nervous system of Mollusca has yet to be seri- tout along its right line of attachment and thrown over to the left side of the animal so as to expose tho organs on ita inner face, a, anus ; i, intestine; r, nephridium (kidney) ; r , aperture of the nephridium ; c, heart ; br, ctenidium (gill- plume) ; pbr, parabranchia ( = the osphradium or olfactory patch) ; x, glandular lanu llae of the inner face of the mantle skirt ; y, acliectal (purpuriparous) gland ; t, testis ; vil, vas de- ferens; p, penis ; mc.columella muscle(muscular j, roces i K ,.f lsllin g the S hell).; y, stomach; 7i, liver. N.B. Note the simple snout or rostrum not in- troverted as a " prob( ously inquired into. The alimentary canal of the Azygobranchia presents little diversity of character, except in so far as the buccal region is concerned. Salivary glands are present, and in
some carnivorous forms (Dolium) these secrete free sul-