114 



MOLLUSCA 



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 

 in many sea-snails 

 (Whelks, (fee.) are 

 very strongly deve- 



, j mif ' r. i Fio. 41. Animal and shell of Ovulum. fc, cephalic 



loped. Ihe Whole tentacles ; d, foot ; h, mantle-skirt, which is natu- 



of this Surface ai)- ral 'y carried in a reflected condition so as to 



. * cover in the sides of the shell. 



pears to be active 



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

 gill-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 Molluscan 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 gill-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 gill- 

 plume we find the so-called 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 F[Q 42 

 ctenidium of the typical Mollusc, 

 which it cannot do on account 



of its position. It should be ', <"> whorls of the shell ; s, . 



,1 . . i> .1 tures. Occupying the axis, and 



to the right OI the anus were exposed by the section, is seen the 



this the case. Recently Spengel "coiumeiia "or spiral pillar. The 



columella " or spir 



upper whor]s of g sldl are seen 



to be divided into separate chain- 



has shown that the parabran- 



chia of Gastropods is the typical 



olfactory organ or osphradium Owen.) 



in a highly-developed condition The minute structure 



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



Fio. 43. Animal and shell of Rostellaria rectirostris. a, snout or rostrum; 

 6, cephalic tentacle ; c, eye ; d, propodium and mesopodium ; e, metapodium ; 

 /, operculum ; &', 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 



Milll. 



Fio. 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 suffi- 

 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 

 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 Flo . 4 5.-ravata 

 from the cords (contrast with Zy- < mouth ; P. operculum ; & 



, , . j T i \ , ctenidium (branchial plume); x, 



gobranchia and Isopleura). At filiform appendage (Trudiment- 



the same time, the junction of ^tfn^cteiild'iumoft ^Tcafform 



the visceral loop above the in- not having its axis fused to the 

 *,.(;,, + n ; !! cu roof of the. branchial chamber is 



testme prevents m all Strepto- the notable character of this 

 neura the shortening of the vis- genus. 

 ceral loop, and it is rare to find a fusion of the visceral 

 ganglia with either pleural, pedal, or cerebral a fusion 

 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.ll2), 

 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-like 

 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 Httorlna ^itloraUs, Lin., re- 

 -, .., moved from its shell; the mantle-skirt cut along 



der-llke arrangement 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 ; br, ctenidium (gill- 

 plume); pbr, parabranchia ( = the osphradium 

 tected (30). The his- r olfactory patch) ; x, glandular lamella of 

 , i c ,\_ the inner face of the mantle-skirt ; y, adrectal 



tolOgy OI the nerVOUS (purpuriparous) gland ; t, testis ; t'rf, vas de- 

 c-iratem nf Afnllnspn ferens ; p.penis; mr,columella muscle(muscular 

 bvsl process grasping the shelly v, stomach; h, liver, 



has yet to be Sen- N.B. Note the simple snout or rostrum not in- 



ously inquired into. troverte<1 as a " P roboscis -" 



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- 



of the two pedal 

 nerves and their lateral 

 branches has been de- 



