Characteristics of the Vertebrata. xxxiii 



hence called ' Acranial/ the neural canal widens considerably in the 

 anterior reg-ion of the body, in correspondence with the increased 

 size of the neural axis it encloses, and with the organs of special 

 sense to which its walls give support. The superior and the central 

 elements of this portion of the axial skeleton make up the skull, 

 and are differentiated from those of the vertebral series, not only 

 by the greater size of the canal they form, but also by the fact that 

 they undergo no segmentation until the stage of ossification is 

 attained to. In the anterior portion, on the other hand, of the 

 lower of the two cavities of the body, segmentation of a character- 

 istic kind is established at an early period of the development of all 

 Vertebrata, by the formation of the vertical 'branchial fissures,' 

 which, in the absence of any anterior prolongations of the peri- 

 visceral cavit}'^, lead directly from the exterior into the digestive 

 tract. The first, and in part the second of these fissures are repre- 

 sented permanently by the Eustachian tube, and the tympanic 

 cavity in all air-breathing Vertebrata possessed of these structures ; 

 the other fissures which are retained in relation with the gill-bearing 

 arches of Fish and Perennibranchiate Amphibia, are, in the higher 

 representatives of the Sub-kingdom, obliterated in the course of 

 their development. 



The integumentary system of Vertebrata may develope either a 

 dermal or an epidermal skeleton, or both ; and muscular fibres are 

 ordinarily interwoven in considerable abundance with its substance. 

 But these structures are never, as in Invertebrata, of primary loco- 

 motor importance, the more deeply-placed skeletal and muscular 

 systems being little less characteristic physiologically than morpho- 

 logically of this sub-kingdom. The jaws of Vertebrata are always 

 modifications of the cephalic parietes, and never, as in Invertebrata, 

 modifications of limbs. No vertebrate animal is aprpctous; the 

 digestive tract has the shape of a distinct and independent tube, 

 except in the region of the cephalic parietes, to which, as also to 

 the branchial arches when present, its walls are adherent. In the 

 abdominal region, the digestive tube is suspended by membranous 

 lamellae, which are never fenestrated except as a consequence of 

 absorption in adult fish, but which do occasionally resemble the 

 membranes with similar functions in Invertebrata by possessing 

 muscular fibres. The digestive tract rarely takes a direct antero- 

 posterior course. Its absorbing and secreting surface is often 



c 



