114 



MOLLUSCA 



foot ; h, mantle-skirt, which is natu- 

 rally yarried in a reflected condition so as to 

 cover m the sides of the shell. 



the renal organ, and consists of a single auricle receiving 

 blood 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 ^__^ .^i^™^^^™-™™™. 

 in many sea^snails '^ *^Tr^^^lire*«sWfe>^^ 

 (Whelks, (fee.) are 

 very strongly deve- 

 loped. The whole ^'Snte,Sfrt'^ ""'^-^''^ °- ° -^■"■- •'' °'^'''- '" 

 of this surface ap- 

 pears to be active 

 in the secretion of a mucous-like substance. The single 

 gUl-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 MoUuscan 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 giU-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 giU- 

 plume we find the so-caUed 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 

 ctenidium 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 

 oKactory organ or osphradium 

 in a highly-developed condition 

 of the epithelium which clothes it, as well as the origin of 



Fio. 42. --Section of the sh'ell of 

 Tiitonium, Cuv. a, apex ; etc, si- 

 phonal notch of the mouth of the 

 shell ; ac to jjc, mouth of the shell; 

 w, iy, whorls of the shell ; s, s, su- 

 tures. Occupying the axis,' and 

 exposed hy 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.) 



The minute structure 



Fio. 43. — ^Animal and shell of Bostellaria reetirostris. a, snout or rostrum; 

 &, cephalic tentacle ; c, eye ; d, propodium and mesopodlum ; 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 



Fia. 44.— Female Janthina, with egg-float (a) attached to the foot; 6, egg- 

 capsules ; c, ctenidium (gill-plume) ; d, cephalic tentacles. 



organ is given off from the superior (original right, see 

 fig. 19) visceral ganglion. 



The figures which are here given of various Azygo- 

 branchia are in most cases suflJ- 

 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 yo, 

 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 f,c,. ^.^vaivata cHstata, Muii. 



o, mouth ; op, operculum ; &r, 

 ctenidium (branchial plume) ; a:, 

 filiform appendage (? rudiment- 

 ary ctenidium). The freely pro- 

 jecting ctenidium of typical form 

 not having its axis fUsed to the 

 roof of the branchial chamber is 

 the notable character of this 

 genus. 



a fusion of the visceral 

 a fusion 



from the cords (contrast with Zy- 

 gobranchia and Isopleura). At 

 the same time, the junction of 

 the visceral loop above the in- 

 testine prevents in all Strepto- 

 neura the shortening of the vis- 

 ceral loop, and it is rare to find 

 ganglia with either pleural, pedal, or cerebral 

 which can and does 

 take place where the 

 visceral loop is not 

 above but below the 

 intestine, e.g., in the 

 Euthyneura (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-hke 

 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 LUtorina littoralis, Lin., re- 

 -" ' moved from its shell; the mantle-skirt cut along 



its right line of attachment and thrown over 

 to the left side of the animal so as to expose the 

 organs on its inner face, a, anus ; i, intestine ; 

 r, nephridium (kidney); r', aperture of the 

 nephridium ; c, heart ; 6r, ctenidium, (gill- 

 plume); p&r, panibranchia (=the osphradium 

 or olfactory patch); x, glandular lamellee of 

 the inner face of the mantle-skirt ; y, adrectal 

 (purpuri parous) gland ; t, testis ; wd, vas de- 

 ferens ; 27, penis ; tmc, columella muscle(muscular 

 process grasping the shell),; v, stomach; A, liver. 

 N.B. Note the simple snout or rostrum not in- 

 troverted as a "proboscis." 



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 MoUusca 

 has yet to be seri- 

 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- 



