Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/674

Rh 646 MOLLUSCA [ZYGOBRANCHIA. products. On account of their position they were termed by him the &quot;capito-pedal orifices,&quot; being placed near the junction of head and foot. Spengel (24) has, however, in a most ingenious way shown that these bodies are the repre sentatives of the typical pair of ctenidia, here reduced to a mere rudiment. Near to each rudimentary ctenidium Spengel FIG. 20. The Common Limpet (Patella vvlgata)in its shell, seen from the pedal surface, x, y, the median antero-posterior axis ; a, cephalic tentacle ; 6, plantar surface of the foot ; c, free edge of the shell ; d, the branchial effe rent vessel carrying aerated blood to the auricle, and here interrupting the circlet of gill lamellffi ; e, margin of the mantle-skirt ; /, gill lamella; (not ctenidia, but special pallial growths, comparable to those of Pleurophyllidia); g, the branchial efferent vessel ; h, factor of the branchial advehent vessel ; i, interspaces between the muscular bundjes of the root of the foot, causing the separate area; seen in fig. 27, e. (Original.) has discovered an olfactory patch or osphradium (consisting of modified epithelium) and an olfactory nerve-ganglion (fig. 32). It will be remembered that, according to Spengel, the osphra dium of Mollusca is definitely and inti mately related to the ,, J gill - plume or cteni- &amp;lt;s s s j dium, being always placed near the base of that organ ; further, Spengel has shown that the nerve-supply of this olfactory organ is always derived from the visceral loop. Ac cordingly, the nerve- supply affords a means of testing the conclu sion that we have in Lankester s capita- Fl - 27. Dorsal surface of the Limpet removed , i i j. ,1 T from its shell and deprived of its black IIJK- pedal bodies the rudi mentary ctenidia. The accompanying dia grams (figs. 34, 35) of the nervous systems of Patella and of Haliotis, as determined by Spen gel, show the identity in the origin of the nerves passing from the visceral loop to SpengePs olfactory ganglion of the Limpet, and that of the nerves which pass from the visceral loop of Haliotis to the olfactory patch or osphra dium, which lies in immediate relation on the right and on the left side to the right and the left gill-plumes (ctenidia) respectively. The same diagrams serve to de till mented epithelium; the internal organs are seen through the transparent body-wall, c, muscular bundles forming the root of the foot, and adherent to the shell ; e, free mantle- skirt ; tm, tentaculiferous margin of the same ; i, smaller (left) nephridinm ; k, larger (right) nephridium ; /, pericardium ; Ix, fibrous septum, behind the pericardium; n, liver; int, intes tine ; ecr, anterior area of the mantle-skirt over hanging the head (cephalic hood). (Original.) monstrate the Streptoneurous condition of the visceral loop in Zygobranchia. Thus, then, we find that the Limpet possesses a sym metrically-disposed pair of ctenidia in a rudimentary con dition, and justifies its position among Zygobranchia. At the same time it pos sesses a totally dis tinct series of ftinc- &amp;lt;? tional gills, which are not derived from the modification of the typical Mollus- can ctenidium. These gills are in the form of delicate FIG. 28. Anterior portion of the same Limpet, with 1 11 / 9fi / the overhanging cephalic hood removed, a, ce- lamelue (ng. -D, J ), phalic tentacle; 6, foot; c, muscular substance which form a Series forming the root of the foot ; &amp;lt;/, the capito-pedal organs of Lankester ( = rudimentary ctenidia); e, extending CO m- mantle-skirt ; /, papilla of the larger nephridium ; nlpfplv rrmnrl thp 9&amp;gt; anus; 7| &amp;gt; P a l illa of the smaller nephridium ; i, celv tue smaller nephridium ; k, larger nephridium ; 7, peri- inner face of the cardium; m, cut edge of the mantle-skirt; 71, j- ,, liver ; v, snout. (Original.) depending mantle- skirt. This circlet of gill-lamella} led Cuvier to class the Limpets as Cyclobranchiata, and, by erroneous identifica- 7 ; tion of them with y x I the series of meta- merically repeated ctenidia of Chiton, to associate the latter Mollusc with the former. The gill -lamellae of Patella are processes of the mantle compar able to the plait- like folds often c e c Jrs.i Pio. 29. The same specimen viewed from the left observed on the front, so as to show the sub-anal tract (ff ) of the F r ,r v larger nephridium, by which it communicates with rc the pericardium, o, mouth; other letters as in fig. 28. chial chamber in other Gastropoda (e.g., Buccinum and Haliotis). They are termed pallial gills. The only other Molluscs in which they are exactly represented are the curious Opistho- branchs Phyllidia and Pleurophyllidia (fig. 57). In these, as in Patella, the typical ctenidia are aborted, and the branchial function is assumed by close -set lamelliform processes ar ranged in a series beneath the man tie -skirt on either side of the foot. In fig. 26, d the large branchial vein of Patella bringing blood from the o-ill series to the honrt Fla 80 - Diagram of the two renal organs (nephridia), to show their relation to the where it Crosses rectum and to the pericardium. /, pa pilla of the larger nephridium ; g, anal papilla with rectum leading from it ; h, papilla of the smaller nephridium, which is only represented by dotted outlines ; I, pericardium indicated by a dotted out- lts.l is seen the series of lamellae there is a short interval devoid of lamellae. The heart in Patella con line, at its right side are seen the two reno-pericardial pores ; /, the sub-anal SIStS of a Single auricle (not tract of the large nephridium given off two as in Haliotis and Fis- near its papilla and seen through the unshaded smaller nephridium ; A s.a, an- SUrella) and a ventricle ; the t( :&amp;gt;-. superior lobe of the large ne- ,, . 1111 phridium; ks.l, left lobe of same; ks.p, former receives the blOOd posterior lobe of same; ks.i, inferior from the branchial vein, the &quot;^visceral lobe of same. (Original.) latter distributes it through a large aorta which soon leads into irregular blood-lacunae.