568 The Animal Kingdom 



by sifting the water in a manner somewhat like that of amphioxus. 

 Many of them had poorly developed appendages and fishlike tails. From 

 the evidence of the remains, it has been concluded that these forms 

 originated in fresh water, but during the Devonian period many grad- 

 ually became adapted for marine life. Thus it is in the fresh waters 

 that the very first vertebrates were spawned, and it is a strange, fish- 

 like form, the ostracoderm, which heralded the new era which the 

 vertebrates were destined to dominate. 



The modern cyclostomes with their jawless mouths and single 

 dorsal nasal openings and lack of paired appendages are perhaps direct 

 descendants of these ostracoderms ; however, they are greatly changed 

 in both appearance and habit. They have attained an eel-like form and 

 and have lost the heavy armor. Perhaps these changes are related to 

 their new role as predatory animals. 



During the Silurian when the ostracoderms were attaining their 

 greatest diversity of form, a new group of primitive fish, the placoderms, 

 arose. These, too, were strange, armored fishlike forms, but unlike 

 the ostracoderms they had jawed mouths. At times these have been 

 termed the "spiny sharks." Despite their possession of a jaw and a 

 more streamlined form, these placoderms represent an evolutionary dead 

 end, and they disappear from the fossil record after the Permian. The 

 placoderms represent an early evolutionary development of a rather 

 efficient predator, but they are not ancestral to the present-day fish. 



The first sharks appear in the fossil record during the latter 

 part of the Silurian. These sharks do not differ too radically in 

 general appearance from our present-day forms. The stalk from which 

 these sharks arose is still unknown ; at any rate, they rapidly increased 

 in numbers during the late Paleozoic and even now occupy an important 

 place in the oceanic communities. During the Mesozoic, skates and rays 

 first appeared as an adaptation to bottom dwelling and mollusc feeding. 



The Osteichthyes first appeared in the early Devonian, thus ante- 

 dating the sharks. It is suggestive from this evidence that bone is 

 earlier in its origin than cartilage, and that the cartilaginous skeleton 

 of the Chondrichthyes is secondarily derived. The bony fish originated 

 in fresh water, and during the Mesozoic they invaded the sea. At the 

 beginning of the fossil record, the two subclasses (Actinopterygii and 

 Choanichthyes) were separate. The Choanichthyes became the dom- 

 inant group of bony fish during the Devonian ; these were the crossop- 

 terygians which later were destined to give rise to the amphibians. Side 



