MOLLUSCA 



109 



products. On account of their position they were termed 

 by him the "capito-pedal orifices," 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. 26. The Common Limpet (PattUa wlgata) in its shell, seen from the pedal 

 surface, z, y, the median an two-posterior axis ; a, cephalic tentacle ; b, 

 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 lamellae ; e, margin of the mantle-skirt ; / gill lamellae (not 

 ctenidia, but special pallial growths, comparable to those of Plenrophyllidia); 

 g, the branchial efferent vessel ; A, factor of the branchial advehent vessel ; 

 i, interspaces between the muscular bundles of the root of the foot, causing 

 the separate areas seen in fig. 27, c. (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 

 gill - plume or cteni- 

 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 capito- 

 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 Spengel's 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- 



I0 - ^- Dors 8 ' surface of the Limpet removed 

 from its shell and deprived of its black pig- 

 mented epithelium; the internal organs are 

 seen through the transparent body-watt, c, 

 muscular bandies forming the root of the foot, 

 and adherent to the shell; , free mantle- 

 skirt ; nn, tentaculiferous margin of the same ; 

 i, smaller (left) nephridium ; t, larger (right) 

 nephridium ; /, pericardium ; Lr, fibrous septum, 

 behind the pericardium; it, liver; int, intes- 

 tine ; KT, 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 func- 

 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 



. . 



the overhanging cephalic hood removed, a, ce- 

 phalic tentacle ; b, foot ; c, muscular substance 

 forming the root of the foot ; d, the capito-pedal 

 organs of Lankester ( = rudimentary ctenidia) ; e , 

 mantle-skirt ; /, papilla of the larger nephridinm ; 

 p, anus ; A, papilla of the smaller nephridium ; i, 



er nephridium ; t, larger nephridium ; I, peri- 

 cardium ; , cnt edge of the mantle-skirt ; *, 



"* (Original.) 



lamellae (fig. 26,/), 

 which form a series 

 extending com- 

 pletely round the 

 inner face of the 

 depending mantle- 

 skirt. This circlet of gill-lamellae led Cuvier to class the 

 Limpets as Cyclobranchiata, and, by erroneous identifica- 

 tion of them with 



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- 

 e * e like folds often 



FIG. 29. The same specimen viewed from the left observed on the 

 front, so as to show the sub-anal tract (/) of the * f *l, K 

 larger nephridium, by which it communicates with roC 

 the pericardium, o, month; other letters as in fig. 28. gjijal 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 gill-series to the heart 

 is seen ; where it crosses 

 the series of lamellae there 

 is a short interval devoid 

 of lamellae. 



The heart in Patella con- 

 sists of a single auricle (not 

 two as in Haliotis and Fis- 

 surella) and a ventricle ; the 

 former receives the blood 

 from the branchial vein, the 

 latter distributes it through a large aorta which soon leads 

 into irregular blood-lacunae. 



FIG. 30. Diagram of the two renal organs 

 (nephridiaX to show their relation to the 

 rectum and to the pericardium. / pa- 

 pilla of the larger nephridinm ; g, anal 

 papilla with rectum leading from it ; A, 

 papilla of the smaller nephridium, which 

 is only represented by dotted outlines ; 

 J, pericardium indicated by a dotted out- 

 line, at its right side are seen the two 

 reno- pericardia! pores ; /, the sub-anal 

 tract of the large nephridium given off 

 near its papilla and seen through the 

 unshaded smaller nephridinm ; fcs.o, an- 

 terior superior lobe of the large ne- 

 phridium ; fctJ, left lobe of same ; kt.p, 

 posterior lobe of same; te.i, inferior 

 sub- visceral lobe of same. (Original. ) 



