PISCES 93 



Class Pisces is more advanced, until it can be split up into its 

 phylogenetic constituent branches, it is best to preserve it as a 

 provisional group of convenience, representing the first Grade of 

 the Gnathostomata. 



Grade I. Class PISCES. 



This, the lowest Class of the Gnathostomata, contains all the 

 true fish. Like the Cyclostomes, they are still adapted to an 

 aquatic life, and preserve many primitive points of structure which 

 must have characterised the early ancestors of all Gnathostomes. 



Throughout life the Pisces breathe by means of gills borne on 

 the visceral arches. The gill-slits are formed by ectodermal pits 

 meeting endodermal outgrowths. But whereas in the Cyclostomes 

 the inner pouches are large, and give rise to the gill-lamellae by a 

 folding of their endodermal lining, in the Pisces the ectodermal 

 pits are relatively more developed. Their gill-slits are closely 

 approximated and transversely elongated ; and open, as a rule, widely 

 both inwards and outwards. The gill-lamellae tend to grow towards 

 the exterior ; and, indeed, arise in the embryo of the higher fish (and 

 Amphibia) as projections of the ectoderm, into which pass branches 

 of the primary aortic arch. But the difference is more apparent 

 than real, and even in these cases the endoderm seems to grow out- 

 wards below the ectoderm, forming a gill-lamella with merely a 

 superficial covering of ectoderm (Greil [185-6]). 



Goette [169] considers that the gill-lamellae of the Cyclostomes 

 and of the Gnathostomes are not homologous (except perhaps the 

 spiracular gill). There can be little doubt, however, that the gills of 

 all the Craniata are really derived from some common origin (Dohrn 

 [1 14«, 1 1 5]). Possibly they are divergent forms from some original 

 less specialised gill in the covering of which both the ecto- and the 

 endoderm took a share (Moroff [303]). The ectoderm spreading 

 inwards seems to have gradually encroached upon the endoderm ; 

 less, however, in the gill-slits of Elasmobranchs than in those of 

 higher fish. 



The structure of a free branchial bar is very constant in the 

 Pisces. Along its inner edge is the skeletal arch (Fig. 57); out- 

 side this in the embryo is the coelomic canal, opening below into 

 the pericardial cavity. From the walls of this canal (lateral plate) 

 are developed the visceral muscles. The primitive aortic arch 

 passes along the posterior face of the bar ; but in the adult it 

 becomes variously broken up into an efferent (branchial vein) and 

 one or two afferent vessels (branchial arteries), communicating with 

 each other through the lamellae. The vessels run up the outer side 



