654 MOLLUSCA [AZYGOBRANCHIA. hump ; upon the upper surface of the metapodium is developed an operculum. Genera : Atlanta, Oxygurus. Probably here belong the Palaeozoic fossils Sellcrophon. Sub-order 2. Carinariacca. Characters. Visceral hump greatly reduced in relative size; shell small, cap-like, hyaline ; ctenidium (branchial plume projecting from the small sub- pallial chamber ; body cylin drical ; of the foot-lobes only the mesopodium is prominent, provided with a sucker, and compressed laterally so as to form a vertical plate -like fin projecting from the ventral surface ; the propodium forms simply the ventral surface <>! the anterior region of the cy lindrical body whilst the me tapodium forms its posterior region. Genera : Carinaria, Cardio- poda. Sub-order 3. -Pterotrachcacea. Characters. Visceral hump still further reduced, forming a mere oval sac embedded in the posterior dorsal region of the cylindrical body ; no shell ; foot as in Carinariacea, except that the sucker is absent from the mesopodium in the females. Genera : Pterotrachea, Firu- Fl - 60. Cariwarta mediterranea. loidcs. the visceral loop of the Natantia is Streptoneurous. Special to the Natantia is the high elaboration of the lingual ribbon, and, as an agreement with some of the Opistho- branchiate Euthyneura but as a difference from the Azygo- branchia, we find the otocysts closely attached to the cerebral ganglia. This is, however, less of a difference than it wa - A. The animal. B. The shell removed. C, D. Two views of the shell of Cardiopoda. a, mouth and odontophore ; I, cephalic tentacles ; c, eye ; d, the fin-like mesopodium ; d , its sucker ; e, metapodium ; /, salivary glands ; h, border of the mantle-flap ; i, ctenidium (gill-plume) ; m, stomach ; n, intestine ; o, anus ; p, liver : t, aorta, springing from the ventricle ; w, cerebral ganglion ; v, pleural and pedal ganglion ; IP, testis ; x, visceral ganglion ; y, vesictila seminalis ; z. penis. (From Owen.) Further Remarks on the Natantia A zygobranchia. Logically the Natantia should stand as we have placed them, viz., as a special branch or section of the Azygobranchia, related to them somewhat as are the Birds to the Reptiles. They are true Azygobranchia which have taken to a pelagic life, and the peculiarities of structure which they exhibit Fio. 49. Atlanta (Oxyc/ura) Kerawlrenii (magnified 20 diameters). , mouth and odontophore ; 6, cephalic tentacles ; c, eye ; d, propodium (B) and meso podium ; e, metapodium ; /, operculum ; h, mantle-chamber ; i, ctenidium (gill-plume); k, retractor muscle of foot; !, optic tentacle; m, stomach; n, dorsal surface overhung by the mantle-skirt, the letter is close to the salivary gland ; o, rectum and anus ; p, liver ; q, renal organ (nephridium)]; s, ven tricle ; u, the otocyst attached to the cerebral ganglion ; w, testis ; .T, auricle of the heart ; y, vesicle on genital duct ; ~, penis. (From Owen.) are strictly adaptations of the structure common to them and the Azygobranchia consequent upon their changed mode of life. Such adaptations are the transparency and colourlessness of the tissues, and the modifications of the foot, which still shows in Atlanta the form common in Azygobranchia (compare fig. 49 and fig. 39). The cylindrical body of Ptcrotrachcacca is paralleled by the slug-like forms of Euthyneura. Spengel has shown that at one time supposed to be, for it has been shown by Lacaze Duthiers, and also by Leydig, that the otocysts of Azygo branchia even when lying close upon the pedal ganglion (as in fig. 21) yet receive their special nerve (which can sometimes be readily isolated) from the cerebral ganglion (see fig. 36). Accordingly the difference is one of position of the otocyst and not of its nerve-supply. The Natantia are further remarkable for the high development of their cephalic eyes, and for the typical character of their osphradium (Spengel s olfactory organ). This is a groove, the edges of which are- raised and ciliated, lying near the branchial plume in the genera which possess that organ, whilst in Firuloides, which has no branchial plume, the osphradium occupies a corresponding position. Beneath the ciliated groove is ml Fia. fJl.Pterotrachea miitica seen from the right side, o, pouch for reception of the snout when retracted ; c, pericardium ; ph, pharynx ; oc, cephalic eye ; g, cerebral ganglion ; g , pleuro-pedal ganglion ; pr, foot (mesopodium) ; r. stomach ; i, intestine ; n, so-called nucleus ; br, branchial plume (ctenidium) ; w, osphradium ; mt, foot (metapodium) ; z, caudal appendage. (After Kefer- stein.) placed an elongated ganglion (olfactory ganglion) connected by a nerve to the supra-intestinal (therefore the primitively dextral) ganglion of the long visceral nerve-loop, the strands of which cross one another, this being characteristic of Streptoneura (Spengel). The Natantia belong to the " pelagic fauna " occurring near the surface in the Mediterranean and great oceans in company with the Pteropoda, the Siphonophorous Hydrozoa, Salpa3, Leptocephali, and other specially-modified trans parent swimming representatives of various groups of the animal kingdom. In development they pass through the typical trochosphore and veliger stages provided with boat-
like shell.