/ 



Ixxiv Introduction. 



four in number. They are usually found to form double comb-like 

 rows upon each branchial arch ; but the last of the branchial arches 

 very commonly fails to develope more than a single row of g-ill- 

 processes ; and not rarely is wholly gill-less. 



This reduction may be accompanied by a similar reduction in the gil^ 

 arch immediately in front, and we find the third arch carrying a uniserial 

 gill in Malthea, whilst it is gill-less in the Cuchia (^Amphijjnous). The 

 fifth branchial arch, which is dentigerous in most, and branchiferous in no 

 Teleostei, lias a uniserial gill developed upon it in the Dipnoi and in 

 Hexanchus. All the Elasmohranchii, the Dipnoi, and the Ganoidei, with 

 the exception oiPolyfterus and Planirostra, have a uniserial gill developed 

 upon the opercular arch anteriorly to the most anterior of the gill-laminae 

 developed in Teleostei. In the Sharks and Rays this anterior gill forms 

 the anterior fixed gill lamina of their anterior gill -pouch ; in the Chi- 

 maerae and Ganoidei, it forms the so-called ' opercular' gill. In the 

 Dijmoi, the development of the opercular gill appears to have prevented 

 that of the two biserial gills placed next posteriorly in typical fish ; in 

 the American species the gills of the third branchial arch appear to have 

 been lost also, whilst they persist in the African, as do those of the fourth 

 and fifth branchial arches in both species. Except in Hexanchus and 

 Heptanchus, there are only five gill-sacs in the Sharks and Rays, the last 

 of which contains only a single gill-lamina disposed upon its anterior 

 wall. This half-gill is homologous Avith the posterior row of the biserial 

 gill developed upon the fourth branchial arch of Teleostei, but it is not 

 represented in the Chimaerae. The pseudobranchia of osseous Fish is 

 homologous with the spiracular pseudoliranchia of Ganoidei and Elasmo- 

 hranchii, and not with their anterior functional half-gill, nor with the 

 thyroid vaso-ganglion, which in many Fish underlies the anterior basi- 

 branchials. In osseous Fish, the pseudobranchia receives arterialized 

 blood from the first branchial efierent vein ; and it sei'ves as a diver- 

 ticular rete mirahile for the eye within which the vessels proceeding from 

 it develope the so-called 'choroid gland.' In the Elasmohranchii, Ga- 

 noidei, and Dipnoi, it serves as a rete mirahile for the brain as well as for 

 the eye, but it has no ' choroid gland' developed in connection with it. 

 Accessory aerating organs which enable the fishes possessing them to 

 support respiration when out of the water, are developed in several 

 genera of Teleostei {Anabas, Saccohranchus, Amphipnous), in relation 

 with the interior of their branchial cavity. 



The morphological identity of the functionally pulmonary air-sacs of 

 the Dipnoi with the air-bladder of an oi'dinary Teleostean Fish, which 



