SKULL — CHONDROCRANIUM 



65 



brae, but are not connected with any other skeletal structures (fig. 

 66). Then the ventral ends of each pair unite and the successive 

 arches thus formed become connected in the midventral line by a 

 series of unpaired cartilages (copiilae). The series of arches begins 

 just behind the mouth and extends posteriorly to just behind the 

 last gill-cleft, clefts and arches alternating. The arches serve to 

 strengthen the digestive tube which is weakened by the clefts, and 

 also as supports for the gills and for attachment of muscles. 



The two anterior arches differ considerably from the rest and have 

 received special names, the first being the mandibular arch, the second 



Fig. 69. — Branchial arches of (A) Heplanchus (Gegenbaur); {B) Chlamydoselachus 

 (Garman) and (C) Cestracion (Gegenbaur). c, ceratobranchial; cbr, cardiobranchial 

 (last copula); e, epibranchial; h, hyale; he, hyoid copula (basihyal) ; m, Meckel's carti- 

 lage; p, pharyngobranchial; 1—7, branchial arches. 



the hyoid arch. The rest are called branchial or gill arches. The gill 

 arches are at first similar to each other, and as they apparently 

 present the condition from which hyoid and mandibular arches have 

 been derived, they are described first. 



Each half of a branchial arch divides early into four cartilages 

 (fig. 69, A), named from above downwards, pharyngobranchial, 

 epibranchial, ceratobranchial and hypobranchial elements, the first 

 lying in the upper lateral part of the pharyngeal wall and the hypo- 

 branchial in its floor, the hypobranchials of the two sides being con- 

 nected by a copula, called a basibranchlal. Gill arches are usually 

 referred to by number, beginning in front, and are limited j^by 



the number of gill-clefts. 

 5 



As a rule the anterior arches are the 



