28 MORPHOLOGICAL TYPES 



specialisation of the skull has not extended very far. The 

 walls of the skull are very incomplete, as is the roof. The 

 notochord extends forwards as far as the forebrain. In the 

 trunk-region, in each segment on each side of the notochord, 

 is a pair of cartilaginous pegs, one behind the other. The 

 anterior peg in each segment (interdorsal) is in front of the 

 ventral nerve-root ; the hinder peg (basidorsal) is in front of 

 the dorsal root. These pegs are the rudiments of the vertebral 

 column, as yet very incomplete and not in any way constricting 

 or interrupting the notochord. 



Between every two gill-slits, behind the last and in front 

 of the first, are cartilaginous rods, the branchial arches or gill- 

 arches. Together they constitute the branchial basket. The 

 most anterior branchial arches, together with cartilages belong- 

 ing perhaps to vanished gill-clefts and others in connexion 

 with the rasping tongue, form a skeletal framework attached 

 to the skull and termed the splanchnocrarsium. The brain- 

 case and sense-capsules are called the neurocranium. In 

 higher forms the term " skull " is usually applied to both these 

 structures, but it should be realised that they are fundamentally 

 distinct. Petromyzon has no biting jaws ; instead, its mouth 

 is round, for which reason the group to which it belongs is 

 known as the Cyclostomes. The splanchnocranium of the 

 Cyclostome is unimportant from the present point of view, 

 because it is so much specialised that it can throw little light 

 on the skulls of higher forms. 



The fins are supported by rays of cartilage. 



Alimentary System. — In order to understand the structure 

 of the alimentary canal and associated organs more easily, it is 

 necessary to leave the adult Petromyzon and to turn to its 

 larval form, which is known as the Ammoccete. The mouth 

 is situated in a buccal cavity separated from the pharynx by a 

 velum. The side walls of the pharynx are pierced by seven 

 pairs of gill-slits. 



Along the floor of the pharynx runs a groove which is con- 

 tinuous anteriorly with a pair of peripharyngeal bands. These 

 rise up on each side of the mouth, behind the velum. Posteriorly 

 the groove runs into a ventral hollow downgrowth of the 



