MOLLUSCA 



117 



hump ; upon the upper surface of the metapodium is developed an 

 operculum. 



Genera : JLtlanta, Oscygunis. Probably here belong the Palieozoic 

 fossils SeUerophon. 



Sub-order 2. — Carinariacea. 



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 venti'al 

 surface; the propodium forms 

 simply the ventral surface of 

 the anterior region of the cy- 

 lindrical body whUst the me- 

 tapodium forms its posterior 

 region. 



Genera : Carinaria, Cardio- 

 poda. 



Sub-order 3. — Fterotracheacea. 



Oharacters. — 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- 

 loides. 



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 was 



PiQ. 60.— Carinaria mediteiranea. A. The animal. B. The shell removed. C, D. Two views of the shell of Cardiopoda. 

 a, mouth and odontophore ; &, 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 ; «, cerebral ganglion ; v, pleural aiid pedal ganglion ; «', testis ; a;, visceral ganglion ; 

 y, vesicula seminalis ; z. penis. (From Owen.) 



Further Remarhs on the 

 Natantia Azygohranchia. — 

 Logically the Natantia should stand as we have placed them, 

 viz., as a special branch or section of the Azygohranchia, 

 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 



Fig. 49. — Atlanta (Oxygura) Keravctrenii (magnified 20 diameters), a, mouth 

 and odontophore ; b, cephalic tentacles ; c, eye ; d, propodium 03) and meso- 

 podium : e, metapodium ; /, operculum ; h, mantle-chamber ; i, ctenidium 

 (gill-plume) ; k, retractor muscle of foot ; I, optic tentacle ; m, stomach ; n, 

 Aoisia surface overhung by the mantle-skirt, tne letter is close to the salivary 

 gland ; o, rectum and anus ; p, liver ; g, renal organ (nephridium)]; s, ven- 

 tricle ; u, the otooyst attached to the cerebral ganglion ; w, testis ; x, auricle 

 of the heart ; y, vesicle on genital duct ; z, 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 Fterotracheacea 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 



<(' ff 



.X 



Fig. 51. — PterotracJiefi, muticct. seen from the right side, a, pouch for reception 

 of the snout when retracted : c, pericardium ; pli, pharynx ; oc, cephalic eye ; 

 g, cerebral ganglion ; gr', pleuro-pedal ganglion ; pr, foot (mesopodium) ; v^ 

 stomach ; i, intestine ; w, so-called nucleus ; &r, branchial plume (ctenidium) ; 

 w, osphradium ; mt, foot (meiapodium) ; z, caudal appendage. (After Kefer- 

 stein.) 



placed an elongated ganglion (olfactory ganglion) connected 

 by a nerve to the supra^-intestinal (therefore the primitively 

 dextral) gangHon 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, 

 Salpse, Leptocephali, and other specially-modified trans- 

 parent swimming representatives of various groups of the 

 animal kingdom. In development they pass through the 

 typical trochosphere and veliger stages provided with boat- 

 like shell. 



