MOLLUSCA 



129 



Jaws and a lingual ribbon are present as in typica 

 Glossophora, the dentition of the ribbon and the number oi 

 jaw-pieces presenting a certain range of variation. Sense- 



C 



't, C 



Fig- 79. Fig. 80. 



Fio. 79. Cawlinia tridentata, Forsk. from the Mediterranean, magnified two 



Ot 



closing the visceral hump ; h, the median spine of the shell. 

 Fio. SO. Shell of Cavolinia trideniata, seen from the side. /, postero-dorsal 

 surface ; g, 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 Suhm 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, z). xnJLU-L 



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 ^_ ^ 



asecoild division (Gym- Flo . si.-Embryo of CW^ridmtato (from 



nosomata), which COD!- Balfour, after Fol.). a, anus; f, median portion 



t 1-11 i f the foot ; pn, pteropodial lobe of the foot 



prises forms highly de- ft, heart ; i, intestine ; ot, otocyst ; g, shell ; r, 



Veloned in rpo-qrH to nephridium ; s, resophagus ; a, sac containing 



1 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 -man tie -skirt, 



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 Limacmidiv ; 

 shell often with con- 

 tracted mouth and di- 

 lated body, its walls 

 sometimes drawn out 

 into spine-likeprocesses, 

 which are covered by 

 reflexions of the free 

 margin of the mantle 

 (Cavolinia, figs. 79, 80). 



Family 1. Cymliiliidse. 

 Genera : Tiedeinannia, 

 Chj.; Haloj:isiji:he, Thc- 

 ceurybia (figs. 82, 83), 

 Cymbulia, P. and L. 

 (fig- 77a). 

 Family 2. Conulariidx 



(fossil). 



Genus : Conularia, Mill. 

 Family 3. Tentaculiltdie 



(fossil). FIG. S3. TJieceimjbia Gandichaudii, Soul., 



Genera : Tentaculitcs, (frni Owen). Much enlarged ; the body-wall 

 Schlth. ; Cornulitcs, 

 Schlth. ; Cokoprion, 

 Sandb. 



Family 4. Hijalcidss. 



Genera : Triptcra, Q. and 



G. ; StyUola, Les. (fig. 



78) ; Balanlium, Lch. ; Vagitulla, Dand. ; Clcodora, P. and 



L. ; Diacria, Gr. ; Plcuropus, Esch. ; Cavolinia, Gioni. (figs. 



79, 80, 81). 



Family 5.T/iccidie. 



Genera : Thr.ca, Low ; Pterothcca, Salt. 

 Family 6. Limacinidaz. 



Genera : Eccyliomphalus, Porti. ; Hderofusus, Fig. ; 

 Spirialis, E. S. (fig. 76) ; Limacina, Cuv. 



Order 2. Gymnosomata. 



Characters. Pteropoda devoid of man tie - 

 skirt and shell ; tentacular processes of the 

 fore-foot well developed and provided with 

 suckers. 



Family 1. Pterocymodoceidse. 

 Genus : Pterocymodoce, Kef. 

 Family 2. Clionidse. 

 Genera : Cliodita, Q. and G. ; Clionopais, Trosch. ; 



Clione, Pall. (fig. 86). 

 Family 3. Pneumodcrmidss. 



Genera : Trichocydus, Esch. ; Spongobranchia, 



d'Orb. ; Pneumodermopsis, Kef. ; Pncumodcrmon, Cuv. (fig. 85). 



removed. a, the mouth ; c, the pteropodial 

 lobes of the foot; /, the centrally-placed 

 hind-foot ; (1, Z, 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 ; y, genital pore ; k, retractor 

 muscles ; o and p, the liver ; H, v, w, genitalia. 



FIG. 83. Shell 

 of Thmury- 



nttosVbt 



lower ' figure 

 shows the na- 



tural size ' 



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 walls of the branchial chamber. 

 The pallial skirt is accordingly well developed and muscular, 

 subserving by its contractions not only respiration but 

 ocomotion. 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- 

 :wet-n the vertical dorso-ventral and the horizontal antero- 

 msterior axes (see fig. 75, (6)). 



B 



