THE ELASMOBRANCH FISHES 



53 



ENDOSKELETON OF ELASMOBKANCHS IN GENERAL 



The endoskeleton in the Elasmobranchs in general varies a great deal in its 

 composition. While in all it is formed of cartilage and never of bone, yet this 

 cartilage differs greatly as to its rigidity. In a shark like Heptanchus macu- 

 latus the cartilage is usually of a clear hyalin type and relatively soft. In H. 

 cinereus, on the contrary, it is strengthened by a deposit of calcium. In some of 

 the more specialized forms calcification is so abundant and so arranged that 

 the cartilage is almost as strong as bone. 

 Cartilage as such in Elasmobranchs is 

 composed of cartilage cells and ground 

 substance. The ground substance consists 

 of a mucin-like substance in which are co- 

 logen fibers. In certain forms elastic fibers 

 may also be included in the cartilage, and 

 calcification is present in many Elasmo- 

 branchs (see Roth, 1911 ) . 



Axial Skeleton 

 SKULL 



The skull in the Elasmobranchs varies 

 considerably in shape. In the rays it is 

 depressed dorsoventrally, while in the 

 sharks in general it is similar in plan to 

 the skuUas described for Heptanchus. In 

 all Elasmobranchs the skull includes the 

 cranium, the capsules for the organs of 

 s])ecial sense, and the visceral arches. 



CRANIUM 



Fig. 58. Development of cranium, 

 Acanthias (Modified from Sewert- 

 zoff.) 



a.c, auditory capsule; asp., alisphe- 

 noidal cartilage; pr., parachordal 

 plate ; tr., trabecular cartilage. 



The cartilaginous cranium or brain case 

 in the Elasmobranchs is unlike the bony 

 cranium of higher forms in that its sides, roof, and floor are welded into a 

 single piece of cartilage. Joined to it also are the auditory and olfactory cap- 

 sules for organs of special sense. The cranium is perforated by nerves and 

 blood vessels which enter or leave the brain. Below the orbit the cranium may 

 project outward as an in£raorl)ital plate (most sharks) or such a plate may 

 be absent (rays). 



The rudiments for the brain case are laid down in the embryo of Acanthias 

 according to Sewertzoff (1897) as three pairs of cartilages (see figs. 58 and 

 70). These are (1) the parachordal plates (j^r.) which lie at the sides of the 

 notochord and posterior to the internal carotid foramina; (2) a pair of ali- 

 sphenoidal cartilages (asp.); and (3) a pair of trabecular cartilages (tr.) 

 anterior to the internal carotid foramina. The parachordal and trabecular 



