600 MOKPHOLOGICAL METHOD AND KECENT PROGRESS IN ZOOLOGY. 



Through the Rhipidoglossa we pass to the Gastropods, which are 

 one and all as3'nimctrical, for even Flssm'eJla^ Patella^ and Dofis, 

 when 3'oung-, develop a spiral shell; while Huxley in 1877 had observed 

 that the shell of Ajdi/sia, in its asymmetry, betra3^s its spiral source. 



The notion, which until recentl}' prevailed, that among these gastro- 

 pods the nontwisted or so-called euthyneurous condition of the visceral 

 nerve cords, as exemplitied by the Opisthobranchs, is a direct deriva- 

 tive of that of the Chitons, has l)een proved to ])e erroneous, since 

 the nerves in Acia^on and ('Jiilina^ like those of the prosoljranchs, are 

 twisted or streptoneurous. And as to the torsion of the gastropod 

 body, recent research, in which one of mv pupils has played a part, 

 involving the discovery of paired reno-pericardiid apertures in llali- 

 otis^ Patella^ and TrocJms^ has resulted in proof that the dextral tor- 

 sion which leads to the monotocardiac condition, does not uniformly 

 affect all organs l3'ing primitively to the left of the rectum, as we have 

 been taught; since, concerning the renal organs, it is the primitivel3^ 

 (pretorsional) left one which remains as the functional kidnev, its 

 ostium as the genital aperture. Nor is the primitively right kidne3^ 

 necessarily lost, for while its ostium remains as the renal orifice, its 

 body, 1)\^ modification and reduction, mav become an appendage of the 

 functional kidnev, the so-called nephridial gland. And we now know 

 there are cases of sinistral torsion of the visceral hump, in which the 

 order of suppression of the organs is not reversed, the arrangement 

 being one of adaptation of a dextral organization to a sinistral shell. 



Though thus specialized and as3'mmetrical as a group, the gastro- 

 ])ods are 3'et plastic to an unexpected degree. Madagascar has 3'ielded 

 a Ph3^sa (/*. lioneUata) with a neomorphic gill, a character shared by 

 species of PlanoiMs {P. corneus and P. marghutfuK), and an Ancylus 

 in which the lung sac is suppressed; while St. Thomas Ishmd has 

 given us a snail {ThyrojjJivrelJa tJtomcnsls)^ the peristome of whose 

 shell is produced into a protective lid. 



In paleontology, history records the fact that in 1801 ITuxle3' 

 o))served that the genus BeJehinHcfi appears to have borne but six free 

 arms, a startling discover^" which la3' dormant till the present 3'ear. 

 And the recent study of the fauna of the great African lakes, in ])ring- 

 ing to light the existence of a halolimnic molkiscan series in Lake 

 Tanganyika, has opened up now possil)ilities concerning the paleon- 

 tological I'osources of enormous a<iu(M:)us deposits, recently discovered 

 in the interior, and has entirelv changed our geological conceptions of 

 the nature of equatorial Africa. 



Time prevents my dealing Avitli other groups, and it nuist suffice to 

 say that witli those ] liave not considered su))stantial woi'k has Ijeen 

 done. From what has ])een said, it is natural to expect that in some 

 dir(>ction or another so vast an accunudation of facts nuist ha\"e 

 extended the Darwinian teaching; and it is now quite clear that this 



