256 rUOGKEDINGS OF TIIK M A I.ACOLOGICAI, .SOCIETr. 



Miss Sollas finds that in all the odontophorous MoUiisca the radiila 

 has an organic basis of chitine ; that in the Docoglossa the teeth 

 further contain as much as 27 per cent, of silica hydrate or opal in 

 their composition, while in the rest of the Gastropoda the chitine is 

 hardened superficially {enamel layer) by deposits containing calcium, 

 iron, and phosphoric acid to the amount of from 2-4 to 6 per cent. ; 

 that the Chitons differ from this second group in alone having ferric 

 oxide as the most important mineral constituent, Avhich causes the 

 dark colour of the teeth. 



By employing Bethe's and other stains the interesting fact is 

 brought out that the various tracts of the radula take the stains 

 differentl)-. A comparison of specimens thus treated should therefore 

 enable a correct correlation to be made of the tracts of raduhc in the 

 several stages of evolutionary development. Miss Sollas' results in 

 this respect, so far as they go, bear out the conclusions set forth in the 

 foregoing paragraphs ; although obviously unaware of the opinion of 

 previous writers on the subject concerning coalescence, she has been 

 almost tempted to suggest that the "marginals are," as she puts it, 

 " multiplied laterals."] 



The circulatory system can be adduced as showing development if 

 those of the tribe at tlie head of the phylum, the dibranchiate 

 cephalopods, and the primitive g.istropods be contrasted. For in the 

 latter the circulatory system, instead of branching off into capillaries, 

 is distended into swollen, irregular cavities, and sinuses, which are, so 

 to speak, insinuated among the various organs of the body, while 

 a certain amount of the blood finds its way back to the heart without 

 passing through the respiratory organs. The Dibranchia, on the other 

 hand, have the most complete circulatory system of any mollusc, the 

 blood being nearly entirely contained in true vessels. 



The molluscan heart, at the same time, offers some anomalies when 

 the different groups are compared. It is most primitive, and more 

 nearly approximates the annelidean type, in Nautilus, where the single 

 ventrical (and no mollusc has more than one) is served by four auricles, 

 whereas in all the other symmetrical mollusca it has but two auricles 

 (except in the Scaphopoda and Aplacophora in which the heart is 

 rudimentary). In the streptoneurous Gastropoda proportionately as 

 the right (originally left) ctenidium becomes aborted in the higher 

 Rhipidoglossa, and disappears in the rest, as the result of the general 

 torsion of the body, so the corresponding auricle diminishes and 

 disappears also. The simplification of the heart in this case, therefore, 

 is not due to any progressive development from a less to a more 

 perfect condition. 



The respiratory system supplies some very interesting points, 

 especially in those cases where a secondary system has been brought 

 into play as in Patella, many Nudibrauchia, and the Pulmonata. 

 With these latter, howbeit, our present enquiry is not concerned, and 

 we confine our attention to the true gills. 



There is every indication that the primitive gill of the mollusca 

 must have consisted of at least a pair of very simple, plume-like 



