280 



FISHES 



CHAP. 



front with the oral cavity. 1 The first of the series of gill-sacs 

 corresponds to the hyo-branchial 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. <J Thus, the outgrowths become converted 



into the lateral halves of a 

 AIO complete ciliated circum-oral 



groove, which is retained 

 even in the Ainmocoetes 

 stage, and recalls the ciliated 

 peripharyngeal ring of Asci- 

 diaiis. Another archaic 

 feature is also to be noted 

 the continuity of the 



Ir.m 



V-ao 



tj.v 



FIG. 162. Petromyzon marinus. Transverse 

 section through the branchial region (semi- 

 diagrammatic), br.m, Branchial membrane ; 

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

 the branchial basket ; d.m, 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- 

 chial basket ; i.a, internal aperture of a 

 gill - sac ; i.jit, inferior jugular vein ; jit, 

 jugular vein (anterior cardinal) ; my, spinal 



111 



groove with a ciliated mid- 

 dorsal pharyngeal Bridge, 

 which has been compared 

 to the " dorsal lamina " of 

 Ascidians, and to the 

 equally characteristic hyper- 

 branchial groove of Amphi- 

 oxus. 3 Ventrally also, the 

 lateral halves of the groove 

 unite to form a single 



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

 muscle of the tongue; r.t, respiratory tube , ,, ._ -,. ,4 



or branchial canal; s, circum - oesophageal tne tnyrc iment, 



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

 ventral cartilage of branchial basket: v.m, ., IT <> j.i 



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



pharyngeal wall as far as 



the last branchial arch. No 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 



1 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. 



2 Dohrn, Mitth. Zool. Stat. Neapel, vi. 1886, p. 49. 



3 Shipley, Quart. J. Micr. Sci. xxvii. 1887, p. 350. 4 Of. p. 343. 



