206 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 craniwm, for the 
enclosure and protection of the brain; (i1.) of sense capsules, 
which fulfil a like function for the auditory, visual, and olfactory 
organs; (i11.) 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, and are situated one on each side of the 
cranial portion of the notochord. The anterior pair or trabeculae 
are pre-notochordal, and he beneath the anterior pertion of the 
brain.| 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- 
' 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. xiil. 1897, p. 413; ibid., Kupffer Festschrift, Jena, 1899, p. 281. 
