100 



LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



The second or hyoid arch is more slender than the mandibular arch to the 

 posterior face of which it is closely applied. It consists of a ventral median 

 piece, the baslhyal; a slender bar, the ceratohyal, on each side of the basihyal; 

 and a stout piece, the hyo mandibular, dorsal to each ceratohyal. The hyoman- 

 dibular articulates to the otic region of the skull and thus acts as a suspensor of 

 the lower jaw. The hyomandibular bears on its posterior margin some slender 

 projections, the gill rays, which in life support the gills. 



The remaining arches, known simply as gill or branchial arches, are similar 

 to each other. Each consists typically of five pieces, named from the dorsal 

 side ventrally : pharyngobranchial, the most dorsal piece, elongated and directed 

 posteriorly; epibranchial, the succeeding much shorter piece; ceratobranchial, 



chondrocranium 

 olfactory capsule 



rostrum — (— 



pterygoquadrate 



cartilage 

 mandibular (first) 



gill arch 



Meckel's 

 cartilage 



intercalary arch 



CE — cenl 



pharyngobranchial 



epibranchial 



ceratobranchial 



hyobranchial 



basibranchial 

 or copula 



hyoid (second) 

 gill arch 



Fig. 33. — Diagram of the chondrocranium, vertebral column, and gill arches of an elasmobranch 

 to show particularly the parts and relations of the seven gill arches. (Slightly modified from Vialleton's 

 Elements de Morplwhgie des Vertebres.) 



another elongated piece; hypobranchial, curved ventral pieces, of which there 

 are but three pairs to the five branchial arches; and the basibranchials, two in 

 number — an anterior small one situated between the medial ends of the first 

 and second pairs of hypobranchials and a large posterior piece between the bases 

 of the fifth ceratobranchials and terminating in a caudally directed point. 

 Epi- and ceratobranchials bear gill rays. Note that the gill arches are not 

 attached to the vertebral column. A diagram of the gill arches is given in 

 Figure 33. 



Draw from the side, showing chondrocranium and visceral skeleton. 



C. THE FORMATION OF THE MEMBRANE BONES OF THE SKULL 



1. Origin of the membrane bones of the skull. — In addition to the chondrocranium and 

 gill arches, still another component enters into the formation of the skull. This component 

 consists of the dermal or membrane bones of the skull. These elements first appear in the 

 ganoid fishes. In these forms it can be observed that the ganoid scales have become fused 

 ■on the head to form large bony plates incasing the head. These scales, which as we have 

 already learned, are dermal in origin, sink into the head and apply themselves closely to the 

 chondrocranium and mandibular arch. It has already been pointed out that in the majority 



