3 82 



THE VERTEBRATES. 



several respective phyla, have independently acquired the same kind of organs, 

 or they pass through similar phases of development. In other words, there appear 

 to be present in the ostracoderms certain latent conditions that produce, sooner 

 or later, the same results in their various descendants, long after the stock is 



Arachnids. 



FIG. 258. Diagram to illustrate the probable phylogeny of the arachnid, ostracoderm, vertebrate stock. 



broken up into separate phyla. For example, in all descendants of the ostraco- 

 derms the dermal skeleton tends to break up into smaller plates which ultimately 

 disappear, leaving the skin naked. In the cyclostomes and in Amphioxus, the 

 dermal skeleton probably disappeared at a very early period in their history. The 

 same thing took place, but at a later period, in the holocephali; still later in the 

 elasmobranchs, teleostomi, and amphibia. Teeth, pectoral and pelvic fins, neural 



