4 ELEMENTS OF MAMMALIAN ANATOMY 



invertebrates and vertebrates. They must have been pres- 

 ent in the Cambrian and Precambrian seas several hundred 

 milHon years ago. They are represented in modern times 

 by the worm-like Balanoglossus, the Tunicates and Bran- 

 chiostoma ( Amphioxus) . 

 III. Vertebrata. Forms with a skeletal axis and dorsal nerve- 

 cord whose anterior end is dilated into a brain. 



3. The Cyclostome Stage 



(A) Cyclostomata. — Fish-like forms having a circular 

 mouth, no jaws present, no paired appendages, skele- 

 ton poorly developed. They make their appearance 

 in the early Ordovician. The modern representatives 

 are the lampreys and hagfishes. 



4. The Selachian Stage 



(B) Gnathostomata. — True jaws, well-developed skeleton, 

 and usually with paired appendages. Include all 

 vertebrates above the cyclostomes. 



a Ichthyopsida (Anamnia): Fish-like forms that breathe, at least 

 while young, by gills; paired appendages, no amnion or allantois. 



I. Pisces (Fish). — Breathe by gills; median and paired 

 fins, except where the latter are lost by degenera- 

 tion. 



(a) Selachii. — True jaws present for the first time; 

 gill slits reduced in number; body covered with 

 plate-like (placoid) scales; notochord strength- 

 ened by blocks of cartilage; general organ sys- 

 tems foreshadowing those of the higher types. 

 A knowledge of the anatomy of selachians 

 is fundamental to a proper understanding of 

 the embryology and morphology of vertebrates 

 in general. Sharks swarmed the Silurian seas. 

 The modern representative of the selachians 

 are the sharks, rays and torpedoes. 



