318 



LOWER VERTEBRATES. 



which differ from each other in the structure of the shoulder-girdle and of the base 

 of the skull. In the first of these, the eustachian canals are prolonged by an osseous 

 roof which covers them in below, until they nearly meet on the middle line of the 

 roof of the pharynx, so that they appear to have but one opening. This sub-order has no 

 tongue, and is hence called the Aglossa. In the next sub-order the structure of the 

 shoulder-girdle is similar to that of the Aglossa, while the eustachian openings are 

 widely separated from each other, and the tongue is present. This sub-order is called 

 the Arcifera, deriving its name from the characteristic peculiarity of the shoulder-gir- 

 dle. This consists in the presence of two longitudinal cartilaginous bands with convex 

 borders which connect the fi-ee and divergent ends of the coracoid and precoracoid 



Fig. 186. — Ccecitia lumbricoidea. 



bones (Fig. 188) ; the cartilage of one side overlapping that of the other. In the next 

 sub-order, that of the Firmisternia, these cartilages are fused into a narrow median 

 mass which intervenes between the adjacent ends of the nearly parallel precoracoid 

 and coracoid bones (Fig 189) the whole structure being incapable of a movement which 



expands and contracts the thoracic cavity, as is the 

 case in the Arcifera. In the last sub-order this 

 region has a structure which is a modification of 

 that of the Firmisternia, but the coracoid diverges 

 much from the precoracoid, and the scapula is 

 articulated to a ball joint of the prootic bone 

 which projects on each side of the skull behind. 

 This structure gives the scapular arch much more 

 solidity than is found in any of the other sub- 

 orders, affording a point d'appui for the pressure 

 exercised by the fore legs in pushing through the 

 earth, for the known species of this division, the 

 Gastrechmia, are burrowers. 

 It is an interesting fact that the frogs of the sub-order Firmisternia have, during 

 an early stage (Fig. 190), the structure of the scapular arch which belongs to the Arci- 

 fera, the consolidation constituting a modification superadded in attaining maturity. 

 Furthermore, young Anura are toothless, and one section of the species with embry- 

 onic shoulder girdle never acquires teeth. So here we have a group which is imperfect 



Fig. 187. — Larva of Typhlotiecfes compressi- 

 cauduSf showing external sac-like gills. 



