458 ELASMOBRANCHII [CH. 



of the pharynx and are horizontal in direction ; the epi- and cerato- 

 branchial stiffen the sides of the pharynx the ceratobranchial 

 being the main portion of the arch, whilst the hypobranchial pieces 

 are found in the ventral wall of the pharynx and converge to imite 

 in a median plate, the basibranchial. To the ceratobranchials 

 are attached a number of thin rods of cartilage which run outwards 

 in the wall of the gill-sac and are called gill-rays. Lying outside 

 the visceral arches are a varying number of cartilaginous rods. 

 Those situated at the sides of the gape are called labial cartilages, 

 those external to the hinder visceral arches extrabranchials (19, 

 Fig. 223). They are equivalent to gill-rays which have become 

 detached from the arches. 



The first gill-slit, called the spiracle, is situated between the 

 jaw and the hyoid just outside the internal ear (Fig. 227). It is 

 a narrow tube, and its use in the more typical forms appears to be 

 to allow vibrations to come more closely in contact with the ear, and 

 to admit the water for breathing. It is often entirely suppressed. 

 The other slits are really flattened sacs, the walls of which are sup- 

 ported by the gill-rays and are raised up into thin folds richly 

 supplied with blood-vessels, which are the true gills. A rudimen- 

 tary gill, the pseudobranch, is sometimes developed on the front 

 wall of the spiracle. No gill is developed on the posterior wall 

 of the last gill-sac. 



In Elasmobranchs we find, as in Cyclostomata, well- developed 



dorsal (or neural) and ventral (or haemal) arches, 

 coTumn bral with their ends deeply embedded in the thick sheath 



of the notochord. This sheath consists partly of the 

 primary sheath secreted by the notochordal cells which has been 

 converted into cartilage by amoebocytes wandering into it and partly 

 of a secondary sheath derived from the sclerotome (v. page 414), and 

 it is divided into separate pieces called centra. Between the centra 

 the sheath remains membranous, and in the middle of each centrum 

 the notochord becomes very much narrowed, so that instead of being 

 a uniform rod it is like a row of beads. The haemal arches meet 

 beneath in the tail, but further forward they stretch out horizontally 

 and become jointed; their outer segments are the ribs, and this is 

 the first appearance of these organs. The ribs project at the level 

 of the centre of the fleshy masses formed by the myotomes. The 

 inner segments of these arches are termed basiventrals. The ribs 

 do not correspond to the outgrowths of the basiventrals in the tail 

 because in one genus of the Osteichthyes (Polypterus] we find two 

 sets of ribs on each side of the vertebral column one above the 



