286 



CEPHALOPODA. 



the anus proves conclusively that there is here no long proctodaeum. 

 Nevertheless, a very considerable part of the alimentary canal has 

 actually, by some authors, been attributed to a proctodaeal invagina- 

 tion. Although we considered this view as disproved, we mention it 

 here as having formerly been, supported by the majority of authors 

 (Metschnikoff, No. 32; Geenachee, No. 14; Ussow, No. 44; 

 Gieod, No. 12 ; Watase, No. 49). 



According to these authors, the structure spokeu of as the enteron was 

 already connected with the ectoderm from its earliest development, and, as 

 this gives rise to the intestine, the latter must therefore have arisen in the 

 form of an ectodermal depression, i.e., as a proctodaeum. Ussow, as well as 

 Girod and Watase, who later investigated this subject very thoroughly, must 

 be regarded as having adopted this view, the details of which are here un- 

 necessary. The hypothetical proctodaeum grows up over the yolk, becoming 

 differentiated in the way described above for the enteron. As to the point to 

 which the proctodaeum extends forward (or the stomodaeum backward) 

 opinions are divided, but according to this view, the liver, tht; caecum and the 

 stomach are all derived from the ectoderm, since the whole alimentary canal 

 is produced by the union of the stomodaeum growing backward and the 

 proctodaeum advancing forward. The whole of the entoderm is represented 

 by the yolk-epithelium and is quite transitory (Watase). Similar statements 

 as to the ectodermal origin of the alimentary canal have been made in 

 connection with other animals (e.g., Insects, Ganin, Witlaczil, Graber, 

 Heymons), but are, in such cases also, altogether improbable, as is proved by 

 the condition of nearly related forms. 



As opposed to the view to which we have just briefly referred, we have given 

 that maintained by RayLankester (No. 29) ; Vialleton (No. 48) ; Bobretzky, 

 (No. 4) and confirmed by ourselves (No. 25). in much earlier stages of de- 

 velopment, as, apart from its greater probability, this derivation of the 

 alimentary canal seems much better founded. 



B. The Covering of the Body and the Shell. 



The ectoderm which covers the body of the embryo seems to pass, 

 with only slight modifications, direct into the body-epithelium 

 (epidermis) of the adult. The shell appears as the secretion of a 

 specially modified part of the ectoderm. In studying the embryonic 

 formation of this organ which is of such importance for the compre- 

 hension of the Cephalopodan body, we are unfortunately restricted to 

 those forms in which it no longer attains full development. Only in 

 a few recent Cephalopods such as Nautilus and Spirula * is the shell 



* [It is very questionable if the shell of Spirula can be regarded as perfect ; 

 it is almost internal and evidently much reduced. — Ed.] 



