62 THE ELASMOBRANCH FISHES 



VISCEEAL SKELETON 



The cartilaginous visceral skeleton in the Elasmohranchs usually consists of 

 seven visceral arches : the mandibular, the hyoid, and five branchial arches. In 

 Chlamydoselachus and the notidanids, however, there are additional branchial 

 arches making a total of eight visceral arches in Hexanchus and Chlamydosel- 

 achus and, as we have seen, nine in Heptanch us. 



Fig. 70. Development of embryonic visceral arches in Acanfhias. (From Sewertzoff.) 

 md.a., mandibular or first arch ; or.2>., orbital process. 



The visceral arches of the adult are more easily understood if we first study 

 their early arrangement in the embryo. In a reconstruction made by Sewert- 

 zoff of the embryo of Acanthias (fig. 70), all the arches appear as bent carti- 

 laginous bars which as yet have not divided into segments. The first or mandi- 

 bular arch (md.a.) takes the form of an inverted U. On the upper (anterior) 

 limb of the arch is a process which abuts against the trabecular cartilage, and, 

 in the adult, forms the orbital process (or. p.) for articulation with the cra- 

 nium. The second or hyoid arch at this stage is sigmoid in shape, and even here 

 joins the cranium in the region of the auditory capsule. The remaining five 

 branchial arches, third to seventh viscerals, are crescentic in shape and like 

 the preceding arches show no signs of segmentation at this stage. 



The mandibular arch in all adult Elasmobranchs, as in Hepfanchus, is di- 

 vided into (1) an upper palatoquadrate (pterygoquadrate) segment (p-q.) 

 and (2) the mandibular segment or Meckel's cartilage (nid., figs. 67 and 69). 

 In simple types like the notidanids the palatoquadrate may have two fairly 

 well defined regions, an anterior palatine (pterygoid) and a posterior quad- 

 rate. In more specialized forms, however, it is impossible to distinguish these 

 segments other than by position. On the anterior part of the quadrate segment 

 may be present an orbital (palatobasal) process w^iich articulates with the me- 

 dian wall of the orbit {Chlamydoselachus, Scymnus, Acanthias) ; or it may be 

 wanting, as for example in Heterodontus (fig. 67) and Raja (fig. 69) where the 

 mandibular arch is shoved forward. The palatoquadrate cartilage unites in 

 front with a similar segment from the opposite side, as does also the mandible. 



