280 FISHES CHAP. 
front with the oral cavity, The first of the series of gill-saes 
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.” Thus, the outgrowths become converted 
into the lateral halves of a 
complete ciliated circum-oral 
eroove, 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 
VG0 vo. iju. Tmt por 
Transverse 
section through the branchial region (semi- 
Fic. 162.—Petromyzon marinus. 
diagrammatic). 67.m, Branchial membrane ; 
and! toysiine 
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.¢, 
fibrous tissue enclosing neural canal; A, 7, 
lateral longitudinal cartilages of the bran- 
chial basket ; ¢.a, internal aperture of a 
gill- sac; 7tjw, inferior jugular véin ; jv, 
jugular vein (anterior cardinal) ; my, spinal 
cord ; nc, notochord ; .ca, neural canal ; 
n.p, neural process ; oes, oesophagus ; p.dr, 
peri-branchial lymph sinus ; 7.m.¢, retractor 
muscle of the tongue ; 7.f, respiratory tube 
or branchial canal; s, circum - oesophageal 
lymph sinus; v.ao, ventral . aorta; v.c, 
ventral cartilage of branchial basket : v.m, 
ventral muscles. (From T. J. Parker.) 
Ascidians, 
equally characteristic hyper- 
branchial groove of Amphi- 
oxus.’  Ventrally also, the 
lateral halves of the groove 
unite to form a_ single 
groove, which, after receiv- 
ing the median aperture of 
the thyroid rudiment,* is 
continued backwards in the 
mid-ventral line of 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. 
* Dohrn, Mitth. Zool. Stat. Neapel, vi. 1886, p. 49. 
* Shipley, Quart. J. Mier. Sci. xxvii. 1887, p. 350. 4 Of. p. 343. 
ee a 
