114 



MOLLUSCA 



o. 41. Animal and shell of Oviilum. h, cephalic 

 tentacles; d, foot; *, mantle-skirt, which is ntitn- 



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, &c.) are 



very strongly deve- 



l J rrnT ' 1, 1 



loped. ine Whole 



of this surface ap- rally 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 Fm _ 42 ___ <* thsheii of 



ctemCUUm Of the typical MolrUSC, Ttitonium, Cuv. , apex;nc, si- 



which it cannot do on account 

 of its position. It should be 



phonal notch of the mouth of the 

 shell ; c to pc, mouth of the shell; 

 *<. f, \v]iorls of the shell ; s, s, sn- 



,1 - . i , r ,1 tares. Occupying the axis, and 



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



- 



this the case. Recently Spengel ^jp^STfiSfiSS, 

 has shown that the parabran- to be divided into separate cham- 

 chia of Gastropods is the typical ^X^'"^ "(S 

 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 



f 



Fio. 43. Animal ami shell of finsttlhtria rcctirnstris. a, snout or rostrum; 

 t, cephalic tent-acle ; c, eye ; </, pi-Mimdmm and nu-snpodimn ; e, metapodinm ; 

 /, operculum ; }i\ proltm-ril sl|ilin;il jmk-h nf 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 



Fio. 44. Female Janthina, with egg-float (a) attached to the foot; ?>, egg- 

 capsules ; c, ctenidium (gill-plnine) ; rf, 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 Flc . 45 ._ ra , mta ,,, M n. 

 from the cords (contrast with Zy- . mouth ; op, operculum ; f>r, 



i -, . i T i \ * ctenidium (branchial plume) : a* 



gobranchia and Isopleura). At filiform appendage (? rudiment 



the Same time, the junction of aryctenidium). The freely pro- 



j xi/i^u v,i )ec t, m g etenulmm of typical form 



the Visceral loop above the in- not having its axis fused to the. 

 tpstinp nrpvpnra in ill t!trov,t,-> roof of the branchial chamber is 



,lme pi events m all btieptO- 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. 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-like 

 nerve-tracts contain- 

 ing both nerve -cells 

 or ganglionic elements 

 and nerve-fibres. Yet 

 in some Azygobran- 

 chia (Paludina) a lad- Fin. 46. Male of Littarina littoralin, Lin., re- 

 T |. n moved fromits shell; the mantle-skirt cut along 



der-llke arrangement jt s right line of attachment and thrown over 

 of the two pedal 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 ; 6?', ctenidium (gill- 

 plume) ; j'/'r, parabraneliia ( = the osphradium 

 or olfactory patch) ; x, glandular htmrlUe of 

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



tolOgy Ot the nerVOUS (purpuripanms) gland; (, lestis; nV, vaa de- 

 ferens; j),penis; mf.colmnella muscle(imiscula 



nerves and their lateral 

 branches has been de- 

 tected (30). The his- 



process grasping the slu'll); p, stomach ; h, liver. 

 .A'./*'. Note the simple snout or rostrum not in- 

 troverted as a " proboscis." 



system of Mollusca 

 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- 



