CHAPTER XX. 

 THE OSTRACODERMS. 



The ostracoderms, as we have seen, constitute a class of animals standing 

 midway between the most primitive vertebrates and the merostome-like arachnids. 

 They are now entirely extinct and only a few representatives have come down 

 to us in the form of fossils that are well enough preserved to afford either full or 

 precise information about them. The recognizable remains usually agree in the 

 minute structure of their exoskeleton, but there is a great diversity in the form 

 and general anatomy of the better known representatives of the class, showing 

 that it must have been a very large one. 



The ostracoderms were not a dominant class. Some of them were nearly 

 or quite blind, their powers of locomotion were limited, and their mouth parts 

 were feeble and ill adapted for attack or defense. In all that constitutes active 

 resourceful animals they were less effective, and estimated for that alone, were 

 less highly organized than their immediate predecessors. The reason for this is 

 obvious enough if we accept the conclusions in the preceding 'chapters, and the 

 fact that they do present this condition is, in itself, important evidence that those 

 conclusions are correct. The ostracoderms, as our theory demands, were transi- 

 tional forms; they were in a phase of structural readjustment that had a definite 

 course to run before either a condition of organic stability could be attained or a 

 high degree of functional adaptation to external conditions could be acquired. 

 They were in, as it were, the pupal period in the phylogeny of the vertebrates. 

 The period was one of suspended efficiency, because great internal changes were 

 taking place and the functional relations of the whole organism to the outer world 

 were necessarily reduced to a minimum. A new type of exoskeleton was form- 

 ing; the lateral eyes, but newly transferred to the walls of a hollow brain, had not 

 fully regained their relations to the outer world; paired enteric diverticula opened 

 to the exterior by newly formed visceral clefts; the oral arches for the first time had 

 been transferred to the haemal surface of the head, a new mouth formed; and the 

 old one closed; and the locomotor functions were about to be shifted from the 

 cephalic appendages to the newly acquired flexible trunk, with its greatly in- 

 creased number of segments. At no other period of organic evolution were so 

 many important internal readjustments taking place at the same time, and no other 

 great class of animals has so quickly run its gamut of changes and so completely 

 disappeared in the process of giving birth to a new race. 



This is as it should be, for it will be seen that the most important events were 

 of the open or shut variety that permit no intermediate stages; when they do occur 

 they at once create totally different conditions, to which the organism must 

 respond by a correspondingly rapid readjustment elsewhere, or go out of existence. 



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