lO VERTEBRATE SKELETON 



Apparently the exoskeleton arose as protective structures in the 

 skin. It never formed large sheets as in Arthropods, because of the 

 different relations of the muscle segments. Its elements are small, 

 separate from each other, some of the Hues between them coinciding 

 with the myosepta which run obliquely to the major axis of the body. 

 Examination of any ordinary fish will show the typical arrangement 



of the scales. 



ELASMOBRANCHII 



Scales. — The placoid scale of Elasmobranchs, the development 

 of which was outlined above, although complex in structure, is 



regarded as close to the primitive ele- 

 ment and as furnishing the starting 

 point from which all other dermal 

 skeletal parts have been derived. 

 Each placoid scale consists of a basal 

 plate of varying size (largest in skates) , 

 Fig. 7.— Longitudinal section of with a hollow spine (fig. 7) directed 

 ^I'puip'cTliiy."' """'"'' ''"'"^^ backwards, arising from its outer 



surface. Basal plate and the inner 

 part of the spine are formed of dentine which differs from bone chiefly 

 in lacking bone cells in most of its substance. The outer surface of the 

 spine is covered by a denser calcified coat (vitrodentine) , commonly 

 called enamel, but not certainly such. The cavity of the spine 

 contains mesenchyme (pulp), blood vessels and nerves. It is fre- 

 quently branched and connects with the corium by one or more 

 openings in the dentine. 



Considerable differences occur in form and size of Elasmobranch scales. 

 They are small in most Euselachii, with rhomboid basal plate and a spine which 

 projects through the epidermis. Most Batoids (skates) have them larger, 

 irregularly placed and have a stronger spine. These reach their extreme in the 

 'teeth' on the sides of the saw of sawfishes (Pristidae). Torpedo, like most 

 electric fishes, lacks all scales. The only scales of Holocephals are those on the 

 peculiar frontal horn and the claspers of the male and in the greatly hypertro- 

 phied spine at the front of the dorsal fin. It is noteworthy that the embryo of 

 Callorhynchus (Holocephal) has rows of scales which lack enamel, upon the 

 occiput and scattered elsewhere. 



Ostracoderms may be mentioned here. The most primitive members of the 

 group have spines without conspicuous basal plates. In other species fusion of 

 similar scales with basal plates have formed the armor covering the anterior 

 part of the body, traces of spines sometimes showing on the outer surface. 



