2O6 FISHES CHAP. 



the haemal arches. To the former belong the ribs of the Elas- 

 mobranchs, and to the latter the ribs of the Teleostomi and 

 Dipnoi. Polypterus alone has both kinds of ribs. 



The Skull. 



The skull is a highly complex structure, the various com- 

 ponents of which are as different physiologically as they are 

 morphologically. It consists (i.) of the cranium, for the 

 enclosure and protection of the brain ; (ii.) of sense capsules, 

 which fulfil a like function for the auditory, visual, and olfactory 

 organs ; (iii.) of certain vertebrae or vertebral elements fused with 

 the hinder part of the cranium ; (iv.) of a series of visceral 

 arches ; and (v.) of a series of paired or median cartilages developed 

 in relation with the mouth and nostrils, which may be collectively 

 spoken of as " labial " cartilages. 



The cranium is formed in the embryo from two pairs of 

 cartilaginous rods or plates, developed in the mesoblast of the 

 head. Of these the posterior pair, or parachordals, underlie the 

 hinder part of the brain, arid are situated one on each side of the 

 cranial portion of the notochord. The anterior pair or trabeculae 

 are pre-notochordal, and lie beneath the anterior portion of the 

 brain. 1 Between their hinder extremities, and in front of the 

 anterior termination of the notochord, is the pituitary body. As 

 development proceeds the parachordals blend with each other 

 and with the trabeculae, while the latter fuse in front to form 

 a median plate the mesethmoid cartilage. The hinder portions 

 of the two trabeculae remain distinct for some time, and enclose 

 between them the pituitary fontanelle, but later they fuse 

 beneath the pituitary body, leaving, however, a pit for its 

 reception the pituitary fossa. Cartilaginous capsules are formed 

 round the cranial sense organs. The auditory or periotic capsules 

 fuse on each side with the parachordals. The optic capsules, 

 either fibrous or cartilaginous, remain free, and do not fuse with 

 the adjacent trabecular region. The olfactory capsules alone are 

 not developed independently, but are formed as lateral out- 



1 As additional primary cranial elements mention may be made of a pair of 

 independently developed " alisphenoid " cartilages, which lie in front of the 

 parachordals between the brain and the eyes, and above the trabeculae, and form 

 a considerable part of the inter-orbital region of the cranium. See Sewertzoff, 

 Anat. Anz. xiii. 1897, p. 413 ; ibid., Kupffer Festschrift, Jena, 1899, p. 281. 



