542 



COMPARATIVE ANATOMY. 



section of the enteric tube. An external branchial duct (br) leads to 

 the exterior. There are several variations in the characters of the tAvo 

 canals which spring from each branchial pouch. The inner ones either 

 open each separately into the digestive tube (Bdellostoma, Myxine) 



(Fig. 305), or they all unite into a median 

 respiratory tube which runs below the diges- 

 tive tube, and being connected in front with 

 the digestive tube carries water to each of 

 the branchial pouches (Petromyzon). The 

 external branchial ducts either open sepa- 

 rately on the sides of the body (Bdello- 

 stoma, Petromyzon), or all the ducts of one 

 side are united into a branchial pore (s) 

 which lies behind the branchial apparatus ; 

 on the left side a special canal (c), which 

 comes from the oesophagus (ductus ceso- 

 phago-cutaneus), also opens into the same 

 pore (Myxine). These different forms may 

 be derived from one another; in the case 

 both of the inner and the outer branchial 

 ducts that condition should be regarded as 

 the primitive one, in which there is a direct 

 connection between the respiratory chamber 

 and the surface ; while, on the other hand, 

 the formation of the respiratory tube, 

 and the union of the external branchial 

 ducts, is the result of a subsequent differ- 

 entiation. 



§ 407. 



Fig. 305. Kespiratory organ 

 of Myxine g 1 u t i n o s a, 

 seen from the ventral sur- 

 face, o (Esophagus, i In- 

 ner branchial clucts. br Bran- 

 chial pouches, br' External 

 branchial ducts, which unite 

 on either side into a common 

 branchial duct, which opens 

 at s. c Ductus cosophago- 

 cutaneus. a Auricle, v Ven- 

 tricle, ah Branchial artery, 

 giving off a branch to each 

 gill, d Lateral wall of the 

 body turned outwards and 

 backwards (after Joh. 

 Miiller). 



In Fishes, the branchial pouches are 

 more closely related to the skeleton. The 

 phasnomena seen in them lead to the con- 

 clusion that each arch of the primitive 

 branchial skeleton carried gills. The upper 

 part of the first (mandibular) arch is not 

 excluded from this ; as is clear from the 

 frequent presence of a gill in the opening, 

 which is found in many Selachii — the so- 

 called spiracular cl_eft — between the first 

 and second arches (mandibular and hyoid 

 arches). The spiracular canal, which repre- 

 sents a degenerate branchial pouch, is suc- 

 ceeded by the true branchial pouches, of 

 which there are, as a rule, five ; and rarely six or seven (Notidani). 

 The wall of the first pouch is supported iu front by the hyoid arch, 

 and behind by the first (i.e. by the third primitive) branchial arch ; 

 the other pouches have just the same characters. In each of them 

 a septum (.<?), supported by cartilaginous rays, extends outwards from 

 the internal branchial skeleton, and serves as the posterior wall of the 



