BEANCHLE OF VEETEBEATA. 541 



The region of the body which is occupied by the branchial cavity 

 corresponds to a head, for the nerves which go to it arise from the 

 myelencephalon in the Craniota. Viewed thus the branchial cavity 

 represents a cephalic enteron. Its nutrient and respiratory signifi- 

 cance is the cause of various differentiations in it, which are partly 

 arrangements which are peculiar to the Vertebrata, and partly 

 arrangements which have been inherited from a lower condition. In 

 addition to the branchial clefts, the ventral groove (hypobranchial 

 groove), which is developed on the ventral surface of the branchial 

 cavity, belongs to the latter series ; this has just the same relations 

 as in the Tunicata (cf. p. 402) ; it is found in the larvae of the 

 Petromyzontes, where it forms a grooved depression, enclosed by 

 ridge-like edges (Fig. 304, h). In Amphioxus this structure is also 

 present. Its presence in various stages of metamorphosis in all 

 Craniota, not only brings these forms into closer connection, but is an 

 indication of their genetic relations to the Tunicata, which must not 

 be forgotten (cf. § 416). 



Branchia3. 



§ 406. 



In the Craniota the branchial clefts are universally much reduced 

 in number, as are also, in correspondence with this, the arches of the 

 branchial skeleton. This phenomenon must be regarded as the 

 degeneration of a primitively larger number of these struc- 

 tures, such as is found in Amphioxus; it is compensated for 

 by the increased size of the surfaces which carry the 

 respiratory vascular plexus. This increase in size is implied by 

 the development of the gills, whereby the blood-vessels, which, in 

 the Acrania, are distributed over a large number of arches, are 

 limited to a less extensive region, and are therefore arranged on a 

 smaller number of arches. The essential point in the formation 

 of branchias in these animals is the increase of the surface which 

 is directed towards the respired medium, and this increase may be 

 effected by means either of lamellre or of cylindrical processes. 

 The branchial arches are provided with various forms of these 

 organs, which enclose the well-developed respiratory vascular network. 

 We find that in the Cyclostomata these organs have special characters, 

 which have but little resemblance to what is found in Amphioxus ; 

 their earliest condition is most like what is seen in the Gnatho- 

 stomata, for the branchial clefts are simply spaces in the body-wall 

 (Fig. 304). They are differentiated into tubes, the median portion 

 of which has its lumen widened, and forms a branchial pouch 

 (Fig. 305, br). Branchial lamellae are raised up from the wall of 

 the branchial pouches in the form of leaf -like folds, in which the 

 respiratory vascular plexus is spread out. Each branchial pouch 

 is connected, by an "internal branchial duct," with the anterior 



