562 CEPHALOCHORDA. 



when the mouth and the first gill-clefts form (Fig. 288), the 

 larval Amphioxus has fourteen primitive segments, the cavities of 

 which, when they arise, are in communication with the archenteric 

 cavity. The primitive segments that form later are yielded by the 

 mesoderm-folds after these have become separated from the entoderm 

 (p. 545). The cavities of these segments are thus from the first quite 

 distinct from the archenteron. The number of body-segments of the 

 adult Amphioxus is approximately sixty-one, and this number is 

 attained even before the appearance of the secondary gill-clefts 

 (Fig. 291). 



The unpaired fin of the adult develops simultaneously with the 

 increase in number of the primitive segments. While the larval fin 

 (Figs. 288, 289, 290, c) consists of a simple thickening of the ectoderm 

 (see p. 544), the cells of which had a columnar form, the adult fin 

 (Fig. 291, c) consists of an ectodermal-fold, into which special parts 

 of the body-cavity (the fin-cavities) extend (Fig. 302 7, 77). Within 

 these fin-cavities the fin-rays develop in consequence of a thickening 

 of the mesoderm which grows up into the cavity from its floor, and 

 these project freely into the cavity (Bay Lankester). 



The unpaired fin extends along the whole of the dorsal side (Fig. 

 291). In consequence of its development, the neuropore (np) is 

 pressed out of its original median position to the left side of the body 

 The anterior end of the unpaired fin passes round the anterior end of 

 the chorda, so that, in the rostral region, the fin is continued also on 

 to the ventral side. This anterior ventral section of the fin, according 

 to Ray Lankester (No. 12), is continued without break into the 

 right oral fold. In passing round the posterior end of the body the 

 unpaired fin is widened and then runs forward along the ventral side 

 of the body as far as to the atriopore, the anal aperture being dis- 

 placed by the developing caudal fin to the left side of the body 

 (Fig. 291). 



While, in the dorsal region, the fin-rays are unpaired, in the ventral 

 region, between the atriopore and the anus, they are paired, though, 

 according to Ray Lankester, they develop in unpaired fin-cavities. 

 The presence of these paired fin-rays has been regarded as an indica- 

 tion that this part of the unpaired fin arose from the fusion of paired 

 folds which represent the backward continuation of the epipleural folds 

 (Ray Lankester, Hatschek). 



The fin-cavities are divided into consecutive compartments (Figs. 

 292, 296, fh), about five of which in the dorsal fin, according to 

 Ray Lankester, belong to one muscle-segment, although a definite 



