THE ELASMOBRANCH FISHES 69 



Since the extraliyoids and extrabranchials botli serve the same purpose the 

 two types may be described together. These structures support the free mar- 

 gins of the gill septa and hence run parallel with the deeper visceral arches to 

 which the septa are attached. They may be present on the hyoid and on all the 

 branchial arches except the last, as in Acanthias and in Raia erinacea {ex.b., 

 fig. 74). In others, while the extrahyoid is lost dorsally it may persist ven- 

 trally, making five inferior and only four superior cartilages (Heterodontus 

 francisci) . In still others, both segments of the extrahyoid arch may be absent, 

 and yet a full complement of extrabranchial arches on the first four branchials 

 may be present (Trygon) . In a reduction of the number of extrabranchials the 

 posterior cartilages are the 

 first to be absent. A fourth ex- ^m^- 

 trabranehial may be lacking 

 ventrally, leaving three below 

 and four above {Scyllium). 



While the extrahyal seg- 

 ments present are normally 

 small, the extrabranchials over 

 the branchial arches may be rig. 75. Sagittal section through a developing verte- 

 well developed. Occasionally ^-'^^' S^^U^^^um canimla. (From Schauinsland.) 

 the tins of the dorsal and ven- chd., notochord; e.e., elastica externa; e.i., elastica 



interna ; ep., chordal epithelium ; iz., inner zone ; ms., 

 tral segments of the anterior middle zone ; oz., outer zone. 



arches overlap as in Hetero- 

 dontus (fig. 71) . In most forms, however, the dorsal and ventral segments fail 

 to touch (Acanthias) , and in many they are relatively insignificant structures 

 (Raia erinacea, fig. 74) . 



We have said above that the extravisceral arch is normally composed of a 

 superior and an inferior segment. In a number of species an interesting con- 

 dition is found in which lateral pieces, extraseptalia, are also added. These 

 may be present as flattened bands of cartilage between the external clefts 

 (Torpedo; Raja clavata) or they may be flattened plates lying underneath the 

 forward projection of the propterygial segment of the fin skeleton as this 

 passes over the region of the gills (Myliohatis) . In Cephaloptera, the devil 

 ray, they are relatively large plates bound under the propterygium. 



Spinal Column 



The spinal column in Elasmobranch fishes shows great variation, from a 

 simple cartilaginous tube around the notochord, as in Heptanchus maculatus, 

 to the highly segmented and calcified column common to many forms. In gen- 

 eral, it consists of a central axis made up of centra upon which is a series of 

 neural arches, which extend throughout the body. In the region of the tail, 

 haemal arches, under the centra, furnish protection for the haemal or blood 

 vessels. A vertebra includes a centrum and its neural and haemal arches. 

 The vertebrae vary greatly in numbers. In a type like Heterodontus there are 

 only a few more than a hundred in the whole column, while in Alopias there 

 are more than twice that number in the tail alone. 



