260 



CEPHALOPODA. 



It is evident from the above that not only do the anterior funnel- 

 folds from the two sides unite to form a common fold but that the 

 posterior folds also fuse with the anterior. At the point of junction 

 of these two sets of ridges, the posterior folds appear as a mere pro- 

 longation backward of the anterior (Figs. 118 and 119). By this 

 process and by the simultaneous extension of the anterior folds, the 

 funnel has already approximated closely to its final shape (Fig. 119). 

 The free edges of the two anterior folds become apposed, but do not 

 as yet fuse (Figs. 141, tr, p. 297 and 143, /, p. 299) and the funnel 



A B 



ma 



ot 



OK. 



FIG. 118. Two embryos of Loligo vulgaris, seen from the posterior or funnel-side 

 (original). %-%, arms; au, eyes; ds, yolk-sac;^, fins ; htf, posterior funnel- 

 folds; k, gills; ma, mantle; ot, otocysts ; rt, retractor of the funnel ; vtf, anterior 

 funnel-folds. Between the two gills (k) is the anal papilla. 



has thus attained the stage which is retained throughout life in 

 Nautilus. In this primitive Cephalopod the funnel, though acting 

 as a tube, is actually formed by the bending round posteriorly and 

 the overlapping of two distinct folds. 



As development advances, the two half-tubes composing the funnel 

 fuse in the middle line (Fig. 120 A) ; the exhalent aperture, however, 

 which could already be recognised at an early stage, being retained. 

 The formation of the funnel is thus completed in all essential points. 

 Laterally, the posterior funnel-folds run to the nuchal cartilage (Fig. 



