ORIGIN AND STRUCTURE OF THE HEAD 83 



iigure of a young Tergipes in which the gonads are shown 

 to consist of paired follicles metamerically arranged and 

 alternating with the paired cerata (reproduced in BRONN's 

 Klassen und Ordnungen, Malacozoa, Plate S5). Very obvious 

 is also the originally segmented structure of the gonads in 

 Davenport's (1893) figures. Alternating with the ento- 

 •dermal pouches, which here bear transverse rows of cerata, 

 the genital follicles are situated, one pair of ovaria and 

 •one pair of testes between every two rows of cerata. 

 I myself was especially struck by the regular alternation of 

 ■cerata and genital follicles, while studying a young Nudi- 

 t)ranch from the harbour of Helder, to which my attention 

 liad been drawn by its perfect transparency. It showed 

 very clearly that the diverticula of the gut, applying them- 

 •selves to the ectoderm in the same way as gill-pouches, 

 cause the formation of the cerata. 



It seems to me not improbable that the gill-pouches of 

 Vertebrates may be directly compared to the gut diverticula 

 of flatworms and Nemerteans and, perhaps, of Nudibranchs. 

 In the axolotl HOUSSAY (1893, p. 33) described and figured 

 very distinctly a double series of lateral diverticula of the 

 •gut reaching from the mouth to beyond the anus and alter- 

 nating quite regularly with the myotomes. Only the fore- 

 most break through and form the gill-slits. His observations 

 for Siredon are confirmed by Miss Platt (1894, p. 933, 961) 

 1or Necturus. These metameric outpocketings are very clearly 

 defined and remind one vividly of the intestinal diver- 

 ticula of the gut in flatworms, which alternate with the 

 genital follicles. On the other hand, I do not think that 

 irom these observations it follows that gill-slits must have 

 been present formerly along the whole length of the soma 

 in Vertebrates, as assumed by B0VERI( 1892). Combining then 

 Meyer's above cited theory on the origin of metamerism 

 (gonocoel theory), with the comparison just made, we have 

 -one more argument in favour of the view that branchiomerism 

 originally corresponds to mesomerism 



In studying ZlEGLERs figures, one gets the impression, 

 Ihat it is fairly obvious, that in Elasmobranchs also the 

 segmentation of the branchial region is simply the con- 

 tinuation of that of the trunk. While in the adult the 

 ■series of gill-slits extends backwards considerably further 

 than the corresponding region of the cranium, causing the 

 glossopharyngeus and the branches of the vagus to take an 



