474 THE PINEAL ORGAN 



rhombic scales on the posterior part of the body, in some examples, e.g. 

 Pteraspis (Fig. 322), were coated on their superficial aspect by an enamel- 

 like layer. In the formation of the chitinous exoskeleton of an arthropod, 

 however, there is a secretion of a cuticular nature from the outer ends of 

 the columnar hypoblast cells or deric epithelium, this becoming condensed 

 forms a hard chitinous shell on the surface of the hypoblast. The shell 

 thus consists of a thickened and hardened cuticle and differs both struc- 

 turally and chemically from enamel and osteodentine. Chitin is a nitro- 

 genous and carbohydrate substance allied in its composition to horn ; 

 it may be impregnated with lime salts, but no true Haversian systems, 

 such as those present in bone, are found in it. Increase in size of the 

 animal including its appendages is obtained by a series of moults (ectdyses) 

 in which the hardened cuticle undergoes softening and is cast off; the 



Fig. 322. — Lateral Aspect of Pteraspis rostrata, an Ostracoderm Fish 

 characterized by the absence of pectoral or pelvic flns, a hypocercal 

 Tail, Large Plates or Scutes covering the Head and Anterior Part 

 of the Body, and Rhombic Scales covering the Remaining Part of the 

 Body and Tail. 



The pineal plate is not perforated in P. rostrata, but in some specimens a 

 pit is present on its internal surface. In P. monmouthensis a complete perforation 

 is found. (E. Ivor White.) 



growth of the animal taking place chiefly in the intervals between the 

 moults ; whereas the increase in size of vertebrates which possess an 

 exoskeleton is similar to that of the skull, a continuous process, taking 

 place partly along the lines of suture between the plates and in the case 

 of dermal bones which have sunk beneath the surface of the skin also 

 by deposit of new bone on the surface of the old, and absorption of bone 

 on the internal surface. 



Another important distinction between the fishes and the palaeostracan 

 arthropods is the existence of median dorsal and caudal fins in the former, 

 as compared with the long, tapering caudal spine of the Xiphosura, as 

 well as the presence in some, e.g. Asterolepis, Remigolepis, Bothriolepis, 

 of a cartilaginous endoskeleton, having the structure of a true pectoral 

 fin inside the bony plates forming the exoskeleton. Finally the existence 

 of a notochord (Fig. 238, p. 341), and the vertebrate position of the heart 

 and main blood-vessels relative to the alimentary canal are fundamental 



