GREGORY, PRESENT STATUS OF ORIGIN OF TETRAPODA 34I 



cation not only gave rise to a more iully protective exoskeleton, but af- 

 forded more rigid bases or fnlora for the attachment of the segmental 

 muscles. 



A further development . of the muscular system, involving superior 

 locomotive powers, rapidly ensued, resulting in a corresponding improve- 

 ment in offensive and defensive adaptations. The primitive food habits 

 required only that microscopic food particles be drawn into the phar}Tix 

 by ciliated tracts and later by the sucking action of the branchial 

 pouches, but these primitive habits were later abandoned for the active 

 pursuit of larger prey. At this stage also the conditions necessary for 

 fossilization were fulfilled, the palseontological record opens with the Si- 

 lurian ostracoderms, and speculation can be replaced by recorded history. 



OSTRACODERMI 



The subsequent history of the head, as thus conceived, has been out- 

 lined above (pp. 318-338). The acquirement of a many-layered skin and 

 of osseous or calcareous deposits in the exoskeleton also conditioned the 

 formation and rapid improvement of accessory locomotive organs, espe- 

 cially the fins. From the beginnings of bilateral S3^mmetry and of the 

 differentiation of the head-end from the tail-end, there must have been a 

 tendency for the locomotive end of the body to protrude behind the ter- 

 minus of the primitive gut, and to become laterally compressed, while the 

 tip of the head-end became either dome-shaped or depressed In the 

 ostracoderms the caudal end is already provided with a web of skin, serv- 

 ing as a sweep and strengthened dor sally by ridge scales. The form of the 

 hard parts was everywhere conditioned by the arrangement of the under- 

 lying myomeres and their connective tissue septa, as shown clearly in the 

 arransrement of the scales in modern fish. Dorsal, ventral and caudal 

 outgrowths of various shapes, strengthened by hard scales or scutes, 

 served to transmit the thrust of the myomeres to the surrounding medium. 



The fusiform free-swimming Birkeniidse while well provided with 

 median or vertical fins were apparently not provided with horizontal or 

 paired fins for steering up and down, this perhaps being effected by twist- 

 ing the body. Thelodus and Lanarkia on the contrary were more or less 

 ray-like in form and the lateral angle or lappet of the body may have 

 assisted in steering up and dovm and in veering or dipping to one side or 

 the other. The Cephalaspidae had a pair of fleshy, scaly flaps behind the 

 cephalothoracic shield, which may have been of considerable assistance 

 in steering. In the Antiarchi the pair of lateral appendages behind the 

 head were protected by a many-layered osseous armor, jointed so as to 

 permit bending, and vaguely suggestive both of arthropod appendages 



