340 THE PINEAL ORGAN 



stomes, and at one time it was thought that the Cyclostomes were a 

 degenerate form of the true fishes. Now, although the older view that 

 the Cyclostomes are degenerate fishes has been replaced by the modern 

 conception that the living Cyclostomes, with their fossil representatives 

 the Ostracodermi, constitute a separate and distinct branch of the phylum 

 Vertebrata, there is no doubt that in many respects the living Cyclostomes 

 are less highly developed than their fossil ancestors. Thus the living 

 lampreys and hag-fishes have no exoskeleton, no pectoral fins, and in the 

 hag-fishes not only are the lateral eyes vestigial and sunk beneath the 

 skin, but the static organ has only one semicircular canal, as compared 

 with two (anterior and posterior) in some of the Ostracodermi, and accord- 

 ing to Studnicka the pineal organ (at any rate in some specimens) is 

 absent altogether. 



In considering the markings found in the head-shields of the sub- 

 class Ostracodermata it is necessary to inquire whether there is anything 

 which corresponds to the head-shield of these most primitive extinct 

 vertebrates among living species of vertebrates. Now Gaskell (1908) 

 showed that the head-shield of the larval Petromyzon closely resembles 

 that of the extinct Palaeozoic fishes (Cephalaspidae), not only in general 

 form (Fig. 135, Chap. 17, p. 191) but in the structure of the mucocartilage, 

 which forms the branchial skeleton and the dorsal and ventral head-plates 

 of the Ammoccetes larva. This is a peculiar type of embryonic cartilage 

 which consists of fibrils whose direction is mainly at right angles to the 

 investing layers of perichondrium ; these fibres are intersected by others 

 which run parallel to the surfaces of the plate. At the points of inter- 

 section of the fibrils are star-shaped cells, and the spaces enclosed between 

 the fibrils are filled with a semi-fluid mucoid material which stains a 

 purple colour with thionin. A somewhat similar structure is seen in the 

 head-shields of the extinct Cephalaspid fishes in which there is an appear- 

 ance of spaces enclosed by osseous laminae running at right-angles to each 

 other. These appearances suggest the existence of a fibro-cartilaginous 

 matrix which, having become calcified, formed a hard plate, different 

 from bone in the absence of definite Haversian systems showing con- 

 centric laminae, surrounding the vascular canals. 



Since Gaskell's time the structure of the head-shields of the Cephala- 

 spids has been studied in detail by Stensio (1927), who describes an 

 exoskeleton consisting of superficial, middle, and basal layers (Fig. 238). 

 The basal layer shows thin fibrous laminae enclosing numerous cell- 

 spaces, the fibres in each lamina being arranged in such a way that they 

 are nearly at right-angles to the laminae next above and below. He differs 

 from Gaskell, however, in that he considers the basal layer is composed 

 of true laminated bone. He further describes minutely the relation of 



