30 STRUCTURE OF THE VERTEBRATES 



of the gill pouches are round, agreeing with the structure of the 

 primitive fossils, but differing from the fish. 



5. Nervous system. The brain is primitive compared with 

 that of the fish, although highly developed compared with 

 Amphioxus. The olfactory lobes arp highly developed, but there 

 is no distinct cerebrum or cerebellum. The other organs of spe- 

 cial sense are poorly developed. The lampreys have two semi- 

 circular canals in the inner ear, agreeing with the fossil forms; 

 the other order (myxinoids) have a single inner ear canal, 

 which is considered by some anatomists to be a degenerative 

 specialization. Most of the cyclostomes are blind, or nearly so, 

 the eyes appearing to be degenerate and not primitive. 



6. Supporting structures. The notochord is covered with 

 sheaths, as in Amphioxus, and is the main support of the body. 

 The vertebrae, which appear first in this class, are minute 

 cartilages which could serve no possible function. A chondro- 

 cranium, or primitive cartilaginous "skull" has developed. It is 

 an open trough without a roof, in which the brain rests. 



7. No bone. The living orders of cyclostomes lack bone en- 

 tirely. The "teeth" of the oral hood are ectodermal horny struc- 

 tures, and have no resemblance nor relationship with the teeth 

 of higher vertebrates. 



8. Nasal opening. There is a single, dorsal, nasal opening 

 connecting the medium with the olfactory sacs. As the tube 

 extends past the olfactory region it is usually called the naso- 

 pituitary opening. 



Classification of the Cyclostomes. The living cyclostomes 

 are divided into two orders which vary widely in development 

 and structure. The fossil forms include three orders which are 

 grouped together as the Sub-class Ostracodermi. From the 

 embryological evidence (Dean) and the fossil evidence (Stensio) 

 each of the living orders may be considered as a sub-class, 

 equivalent to the Ostracodermi. 



With the cyclostomes is often included a group of very small, 

 extinct, animals known from a single genus (Palaeospondylus). 

 If it is an adult rather than a larval stage, it gives evidence 

 of the early segmentation of the notochordal supporting 

 structures. 



