88 VERTEBRATE ZOOLOGY 



2. The mouth is round and is closed only b}' the end of the " tongue." 

 faj There is a median unpaired nostril, and the nasal passage in 

 some (hag-fishes) opens into the mouth; in others (lampreys) 

 it ends blindly beneath the brain. 



4. The branchial clefts are in the form of pouches or pockets, a char- 



acter that has given to the group the name " Marsipobranchii." 

 The number of clefts is, in general, greater than in fishes. 



5. There are no paired appendages. 



6. There are no scales^ 



7. The lateral line organs lie in open grooves. 

 Internal features: 



1. There is, in lieu of a masticatory apparatus, a complex rasping 

 apparatus, called a "tongue", that is armed at the mouth end with 

 chitinous teeth, is supported by several cartilages, and is worked by 

 a specialized set of muscles. Since both hags and lampreys have this 

 apparatus, in spite of their very different modes of life, it is likely 

 that this is a very old character and represents an evolutionary ex- 

 periment even more ancient than does the jaw apparatus of fishes. 



fy The notochord is a persistent unbroken rod much like that of 

 Amphioxus, but extends only to the hind-brain and is covered with 

 an extra sheath of connective tissue. 



S\ The vertebra are represented by primitive neural arches^quite 

 separate from the notochord. 



4. The brain is small but typically vertebrate in structure, with 

 the vagus nerve not included in the cranial region. 



(5) The cranium is entirely beneath the brain and fqrms neither 

 sides nor roof for the latter. Attempts have been made to homologize 

 the cartilages of this cranium with those of the embryonic fish 

 ill. 



The gonads give off their products into the ccelom. Eggs and 

 sperm make their exit directly to the exterior through genital pores 

 situated near the urinary opening. 



7. The inner ear is more primitive than in fishes, having only one 

 or two semicircular canals. 



8. The afferent branchial arteries go directly to the gill pouches in- 

 stead of into the arch between two gill clefts. 



9. A branchial basket composed of cartilaginous bars and rods forms 

 a support to the branchial part of the pharynx. This cannot suc- 

 cessfully be compared with the branchial arches of fishes. 



