2964 THE JOINTED ANIMALS 



veniently vague title for all the animals that have not acquired the characteristics 

 of the Chordata. This change of opinion has been brought about by the attain- 

 ment of a far more intimate acquaintance with the structure and development of the 

 lower animals than our predecessors, with their less refined methods of investigation, 

 could possibly possess; and it has resulted in the splitting up of the so-called in- 

 vertebrates into a number of subkingdoms, each of which is equivalent to the 

 entire group of Chordata. 



It must not, however, be supposed that no advance has been made of late years 

 in chordate morphology, and that the conception of the essential characteristics of 

 the group is the same as it was in the earlier part of the century. So far indeed is 

 this from being the case, that the zoologists of those days would certainly be 

 greatly puzzled to understand the reasons for the present wide extension of the 

 group to embrace such forms as the sea squirts and the worm -like Balanoglossiis, 

 which have no vertebral column, and do not even present the outward semblance 

 of any of the classes of the true Vertebrata. Strictly speaking, therefore, they are 

 not Vertebrates at all; yet their claim to be ranked in the same great category 

 of animals as the lancelet, which also has no backbone, and the fishes is now gen- 

 erally accepted, and is based in the main upon their possession, in common with all 

 the true Vertebrates, of three characteristics not found in any other group of the 

 animal kingdom. These are, firstly, the presence of slits in the lateral walls of the 

 pharynx, by means of which the anterior part of the alimentary canal is put into 

 communication either with the body cavity or directly with the outer world; 

 secondly, the existence, either as a temporary or permanent structure, of a cartila- 

 ginous rod, the notochord, lying lengthwise in the upper part of the body; and 

 thirdly, the position of the principal nervous tract, also in the upper part of the 

 body, but above the notochord. The fate of the notochord in the different 

 classes of Chordates is somewhat varied. In some of the sea squirts, for 

 instance, it persists only in the tail, which may entirely disappear when the 

 animal settles down to its sedentary life. Hence these creatures are sometimes 

 called the Urochordata, or rod tailed. In the lancelet, however, this structure 

 remains throughout life, and extends from the end of the tail to the extremity 

 of the head. Hence the section containing this little fish-like creature is 

 called Cephalochordata, or rod headed. In all the higher members of the assem- 

 blage, however, that is to say, in fishes, amphibians, reptiles, birds, and mam- 

 mals, the notochord falls short of the front end of the head, terminating just 

 behind a point which in the floor of the skull eventually becomes the pituitary 

 fossa. Moreover, in all the forms that acquire a bony skeleton, this rod is to 

 a greater or less extent replaced by the bodies, or centra of the vertebrae, or 

 segments composing the backbone; these centra supporting the bony arches 

 developed for the protection of the dorsal nerve chord. No less varied is the fate 

 of the pharyngeal slits, or visceral clefts. Whereas in the lower Vertebrata, such 

 as fishes, these remain as the branchial slits, in the adults of the more highly 

 organized forms, like mammals, they practically disappear, one only remaining as 

 the eustachian tube, by means of which the back of the mouth communicates with 

 the inner chamber of the ear. 



