28o 



FISHES 



CHAP. 



fr^jny 



front with the oral cavity.^ The first of the series of gill-sacs 

 corresponds to the hyo-brauchial or hyoidean cleft of Elasmo- 

 branchs and other Fishes. Spiracles are absent in the adult, but 

 in the embryo are represented by pouch-like outgrowths of 

 the hypoblast of the oral cavity, which subsequently undergo 

 singular changes.^ Thus, the outgrowths become converted 



into the lateral halves of a 

 complete ciliated circum-oral 

 groove, which is retained 

 even in the Ammocoetes 

 stage, and recalls the ciliated 

 peripharyngeal ring of Asci- 

 dians. Another archaic 

 feature is also to be noted 

 in the continuity of the 

 groove with a ciliated mid- 

 dorsal pharyngeal ridge, 

 which has been compared 

 to the " dorsal lamina " of 

 Ascidians, and to the 

 equally characteristic hyper- 

 branchial groove of Amfhi- 

 oxus.' 



ij.ii rin.t P-I>r 



Fig. 16'2. — Fetroinyzou inarinus. Transverse 

 section through the branchial region (semi- 

 diagrammatic), br.m. Branchial membrane ; 

 d.ao, dorsal aorta ; t^. c, dorsal cartilage of 

 the branchial basket ; dj(i^ dorsal muscles ; 

 e.a, external aperture of a gill -sac ; f,t^ 

 fibrous tissue enclosing neural canal ; h, i, 



lateral longitudinal cartilages of the bran- OXUS. Veiltrally alsO, the 

 chial basket; i.« internal aperture of a ^^^^^^^ j^^j^^^^ ^f ^j^g ^.^^^.^ 

 gul - sac ; i.jio, inferior jxigular v6m ; ju-, o 



jugular vein (anterior cardinal) ; viij, spinal unite to forill a single 

 cord ; 7ic, iiotochord ; n.ca, neural canal ; i • i r^ 



H.p, ieural process ; oes, oesophagus ; p.br] grOOVe, whlch, after receiV- 

 peri-branchial lymph sinus ; 7:m.t, retractor ing the median aperture of 

 muscle of the tongue; r.t, respiratory tube , ,, . i -. . f 4 • 



or branchial canal; n, circum - oesophageal tne tnyrOlCt rudimeut, IS 

 lymph sinus ; v.ao, ventral aorta ; v.c, continued baclvWards in the 

 ventral cartilage of branchial basket ; v.iii^ . , it x- i 



ventral muscles. (From T. J. Parker.) mid - Ventral line 01 the 



pharyngeal wall as far as 

 the last branchial arch. Xo trace of these ciliated structures is, 

 however, to be met with in the adult. 



The branchial lamellae are represented by a series of vascular 

 horizontal and parallel ridges radiating outwards along the roof 

 floor, and lateral walls of each gill -sac, and invested by an 



^ In the Ammocoetes stage the gill-sacs open directly into the larval pharynx, 

 which is retained as the branchial canal, the oesophagus of the adult being an 

 independent and later formation. 



^ Dohrn, Mitth. Zool. Stat. Nca-pel, vi. 1886, p. 49. 



2 Shipley, Quart. J. JUicr. Sci. xxvli. 1887, p. 350. * Cf. p. 343. ■ 



