666 MOLLUSCA [SIPHONOPODA. Jaws and a lingual ribbon are present as in typical Glossophora, the dentition of the ribbon and the number of jaw-pieces presenting a certain range of variation. Sense- c Fio. 79. Cavolinia tridenlata, Forsk. from the Mediterranean, magnified two diameters (from Owen), a, mouth ; ?), pair of cephalic tentacles ; C, C, ptero podial lobes of the mid-foot ; d, median web connecting these ; e, e, processes of the mantle-skirt reflected over the surface of the shell ; g, the shell en closing the visceral hump ; h, the median spine of the shell. Fio. 80. Shell of Cavolinia trirlentata, seen from the side. /, postero-dorsal surface ; <;, antero-ventral surface ; h, median dorsal spine ; i, mouth of the shell. organs are present in the form of cephalic eyes in very few forms (Cavolinia, Clione, and in an undescribed form dis covered by Stihm during the "Challenger" Expedition) ; oto- cysts are universally present. The osphradia are present in typical form, although the ctenidia are aborted ; only one osphradium (the right of the typical pair) is present (fig. 87). The gonads are both male and female in the same individual. The genital aperture is single. Copulatory or gans, often of consider able size, are present (fig. 86, 2). KH JLL/Ji ^^ip-W-y-g The mantle -skirt is present in one divi sion of the Pteropoda (Thecosomata), and in these an extensive sub- pallial chamber is de veloped, the walls of which in the absence of ctenidia have a branchial function. In a second division (Gym- FIC. 81. Embryo of Cavolinto tridentata (from JlOSOmata). which com- Balfo r - af tr Fol.). a, anus ; /, median portion ,. , . , , , of the foot ; pn, pteropodial lobe of the foot ; prises iorms highly de- 7i, heart ; i, intestine ; of, otocyst ; q, shell ; r, volnnpr) in rpo-arrl tn nephridium ; s, oesophagus ; <j, sac containing nutritive yelk; mb, mantle-skirt; me, sub- the processes Of the pallial chamber ; Kn, contractile sinus. fore-foot, the mantle-skirt is aborted. A shell is developed on the surface of the visceral hump and mantle-skirt of the Thecosomata, whilst in the Gymnosomata, which have no mantle-skirt, there is in the adult animal no shell. The embryo passes through a trochosphere and a veliger stage (fig. 81), provided with boat -like shell, except in some Gymnosomata in which the Trochosphere with its single velar ciliated band becomes metamorphosed into a larva which has three additional ciliated bands but no velum (resembling the larva of the Scaphopod Dentalium) ; this banded larva does not form a larval shell (fig. 84). The Pteropoda are divided into two orders. Order 1 . -Thecosomata. Characters. Pteropoda provided with a mantle-skirt, Fio. 82. Theceurybia Gaudichaudii, Soul., (from Owen). Much enlarged ; the body- wall removed, a, the mouth ; c, the pteropodial lobes of the foot; /, the centrally -placed hind-foot ; d, I, e, three pairs of tentacle-like processes placed at the sides of the mouth, and developed (in all probability) from the fore-foot ; o , anus ; i/, genital pore ; k, retractor muscles ; o and p, the liver ; 11, v, w, genitalia. and with a delicate hyaline shell developed on the surface of the visceral hump and mantle-skirt ; visceral hump, and consequently the shell, spirally twisted in one family, the Limacinidae ; shell often with con tracted mouth and di lated body, its walls sometimes drawn out into spine-like processes, which are covered by reflexions of the free margin of the mantle (Cavolinia, figs. 79, 80). Family 1. Cymtuliidae. Genera : Tiedemannia, Chj. ; Halopsyche, Thc- ceurybia (figs. 82, 83), Cymbulia, P. and L. (fig. 77a). Family 2. Conulariidse. (fossil). Genus : Conularia, Mill. Family 3. Tcntaculitidse (fossil). Genera : Tentacitlites, Schlth. ; Cornulif.es, Sehltli. ; Colcopricm, Sandb. Family 4. Hyaleidaz. Genera : Triptcra, Q. and G. ; Styliola, Les. (fig. 78) ; Balantium, Lch. ; Vaginella, Dand. ; Cleodora, P. and L. ; Diacria, Gr. ; Pleuropus, Esch. ; Cavolinia, Gioni. (figs. 79, 80, 81). Family 5. Thecidse. Genera : Thc.cn, Low ; Ptcrothcca, Salt. Family 6. Limacinidse,. Genera : Eccyliomphalus, Porti. ; Heterofusus, Fig. ; Spirialis, E. S. (fig. 76) ; Lima-cina, Cuv. Order 2. Gymnosomata. Characters. Pteropoda devoid of mantle - skirt and shell ; tentacular processes of the fore-foot well developed and provided with suckers. Family 1. Ptcrocymodoceidse. Genus : Pterocymodocc. Kef. Family 2. Clionida. Genera: Cliodita, Q. and G. ; Clionopsis, Trosch. ; Clione, Pall. (fig. 86). Family 3. Pneumodermidss. Genera : Trichocyclus, Esch. ; Spongobranchia, d Orb. ; Pncumodcrmopsis, Kef. ; Pneumodcrmon, Cuv. (fig. 85). Branch b.SIPHONOPODA. Cephalopoda in which the two primarily divergent right and left lobes of the mid-region of the foot have their free borders recurved towards the middle line, where they are either held in apposition (Tetrabranchiata), or fused with one another to form a complete cylinder open at each end (Dibranchiata). This fissured or completely closed tube is the siphon (fig. 75, (6), mf) characteristic of the Siphono- poda, and is used to guide the stream of water expelled by the contractions of the w r alls of the branchial chamber. The pallial skirt is accordingly well developed and muscular, subserving by its contractions not only respiration but locomotion. The visceral hump is never twisted, and ac cordingly the main development of the pallial skirt and chamber is posterior, the excretory apertures, anus, and gills having a posterior position, as in the archi-Mollusc. At the same time the visceral hump is usually much elon gated in a direction corresponding to an oblique line be tween the vertical dorso-ventral and the horizontal antero- posterior axes (see fig. 75, (6)). FIG. 83. -Shell bici norfolk- ensis; the lower figure shows the na
tural size.