DEVELOPMENT OF THE EMBRYONIC RUDIMENT OCTOPUS. 265 



that the Octopoda must be regarded as derived and the Decapoda as 

 primitive forms. The conjecture as to the nature of these promin- 

 ences is further rendered probable by the fact that fins occur in some 

 adult Octopoda (Octopus membranaceu*, I '/'// //octopus, Cirrhoteutk!e). 



The oral aperture appears in Octopus at a very early stage and 

 soon takes the shape of a semicircular groove bordering a swelling. 

 The appearance of the rudiments that lie on the ventral side closely 

 resembles that described for Loliyo and will best be understood by 

 a study of Fig. 122 A and R, and by comparing this figure with the 

 illustrations of the Loiiyo embryo given in Figs. 117 and 118. 



It should be noted that the funnel forms, in the Octopoda, in the 

 same complicated way as in Loligo and the other Decapods. A paired 

 posterior fold appears and, in the course of development, unites with 

 an anterior fold which is also paired to form the funnel and its lateral 

 parts (Figs. 122 and 12.",, l.tf, vtf and //). 



I-'KT. 122. Two embryos of Octopus Tidyari* at different ages, seen from the posterior 

 sidf (original). (ir, arms ; a-u, optic swellings ; ds, yolk-sac ; htf, posterior funnel- 

 fold ; /:, gills ; ///, mantle ; ot, otocysts ; vtf, anterior funnel-fold. 



In comparing Loligo and Octopus, we are struck by the fact that, in the 

 latter, the separate organs appear very early, but do not develop further with 

 corresponding rapidity, so that, as contrasted with organs that appear later, 

 their development is retarded. This peculiarity was pointed out by BROOKS 

 (No. 7), in comparing the form examined by him (Loligo Pealii) witli 

 (H; KNACKER'S Cephalopodan embryo. In Octopus, for instance, the rudi- 

 ments of the arms appear early, their number being complete even earlier 

 than the stage depicted in Fig. 122 A, but they then develop very slowly. 

 With regard to the order in which the arms appear, this seemed to us to be 

 the same as in Loligo, i.e., from the rudiment of the funnel towards the 

 mouth. Two pairs of arms appear as slight swellings in front of the optic 



