THE GILLS. 305 



arms, a similar strand running from the posterior part of the ganglion to the 

 funnel. The latter, the pedal nature of which can hardly he doubted, thus 

 (according to Pelseneer), at this early stage, receives its nerve-elements from 

 the same ganglion as the arms. 



Another argument in favour of the pedal nature of the arms is found in the 

 statement of Jatta (No. 18) that the brachial nerves have their roots in the 

 pedal ganglion and that they do not pass into the cerebral ganglion as has 

 been stated, Pelseneee further rightly points out that the arms in their 

 origin (and also in the manner of their innervation) are clearly ventral 

 organs, and those that assume a dorsal position only reach it in a late stage 

 of development (p. j!63). Only as the arms are displaced anteriorly, and to 

 some extent dorsally, does the part of the central nervous system innervating 

 them and originally belonging to the pedal ganglion shift in a dorsal direc- 

 tion (Peeseneer, No. 38). J. Steiner's physiological researches,* according 

 ro which the destruction of the pedal ganglion led to paralysis of the arms 

 and thus proved them to belong to the foot-region, are also significant in this 

 connection. 



We thus have strong reason for regarding the arms, not as cephalic ap- 

 pendages, but rather as gi - eatlv modified parts of the foot (cf. Chap. XXXIV. ).t 



E. The Cartilaginous Skeleton. 



The various parts of the cartilaginous skeleton are regarded by some 

 authors (Metschnikoff, Ussow) as ectodermal structures which have gradu- 

 ally shifted inward. Bobretzky's figures of the optic cartilage seem to 

 confirm this method of origin which at first sight is surprising. Around each 

 eye (Fig. 143 ad, adk), the ectoderm is seen to become greatly thickened ; these 

 thickenings, in consequence of the development of the folds described above, 

 come to lie deeper. Even if these figures appear convincing, we must not 

 exclude the conjecture that some part of the superficial cell-layers may 

 have already become differentiated, and in this case might be reckoned as. 

 mesoderm. 



F. The Gills. . 



The "ills were found to arise as papilla-like prominences in front 

 of the mantle near the anal region (Fig. 85 C and D, k, p. 189), to 

 be gradually grown over by the latter (Figs. 87 arid 88 /!. p. L94), 



Sitz.-Ber. Akad. Berlin, 1890. 

 1 j More recently Kerr (No. II.) has opposed the pedal nature of the arms of 

 Nautilus and reverted to the older view of Grobben that they are cephalic. 

 While advancing several weighty arguments against some of the evidence 

 which has been put forward regarding their pedal nature, he practically gives 

 no striking reasons in favour of their cephalic nature and we think that at 

 present, at any rate, we cannot unhesitatingly accept his conclusions. 

 The cmestion seems to rest on the justification of the attempts to homologise 

 the anterior sub-oesophageal ganglionic mass of the Cephalopoda with one of 

 the highly differentiated ganglia of the Prosobranchia. — Ed.] 



X 



