528 Causes and Course of Organic Evolution 



saltatory, as in Adineta raga amongst rotifers, and Cardium 

 amongst lamellibranchs, or discoid and creeping, or reduced 

 in size and function, or absorbed as in Triarthrida of the lower, 

 and Ostrcea of the higher group. 



The cement gland evolutionarily is even more suggestive 

 here than for Gasteropoda. Thus (173: 42): "A pit-like de- 

 pression of the ectoderm is found on each side of the middle 

 line, lying exactly above the mass of cells from which the pedal 

 ganglion develops. This is the paired rudiment of the byssal 

 gland. These two depressions together with the whole of 

 the ectoderm lying between them soon sink in deeper, having 

 then only one common aperture; their epithelium becomes 

 modified into glandular cells which secrete the well known 

 byssal threads that serve for the attachment of the embrj^o 

 or young lamellibranch. As the body grows further the paired 

 rudiment of the byssal gland is drawn further and further 

 inward, and finally opens out tlirough a long duct with a nar- 

 row aperture. This gland degenerates later (in Cyclas), and 

 in the adult is a mere sac-like vestige. In other lamellibranchs, 

 on the contrary, as is well known, it functions throughout 

 life and is very highly developed." 



The secretion of this gland functions also in molluscs for 

 attachment to foreign objects as in some rotifers, but similarly 

 the secretion may be poured out round the mollusc, as in Modi- 

 ola, Dacrydium, and others, to form a viscuous envelope in 

 which extraneous particles may be lodged (174, II' 913). So 

 far as the writer is aware no faint trace even of a pedal ganglion 

 has been observed in most rotifers, but there seems a possi- 

 bility that such may exist in several genera. For the detailed 

 description given by Zelinka in the case of Discopus (175: 

 353), as well as the accompanying illustrations, alike demon- 

 strate the existence of a pedal ganglionic system, the close 

 similarity in the nervous system of that genus and of lamelli- 

 branchs, and an exact relation between the pedal ganglion 

 in Discopus and the adhesive gland. The origin of the gland 

 also, as a pair of ventral swellings, is noteworthy. 



The absorption of the mastax in development of the lamelli- 

 branchs from a rotiferan type seems wholly correlated with 

 the soft nature of their food. The close similarity of the ali- 

 mentary canal, of the larval eyes, of the nerve distribution, 

 and not least of the excretory organs in larval stages, need 

 merely be indicated. 



As regards Cephalopoda the remarkably early and abundant 

 development of this group pala?ontologically has often been 

 commented on. It would therefore have been of great in- 



