2 INVERTEBRATES. 



possess ; and it has resulted in the splitting up of the so-called invertebrates 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 characters 

 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 Balanoglossus, 

 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 generally 

 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 communica- 

 tion either with the body-cavity or directly with the outer world ; secondly, the 

 existence, either as a temporary or permanent structure, of a cartilaginous 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 some- 

 what 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 assemblage, however, that is to say, in fishes, amphibians, reptiles, 

 birds, and mammals, the notochord falls short of the front end of the head, ter- 

 minating j ust 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 vertebras, 

 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 

 organised 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. 



With this brief resume of the fundamental features of Chordate morphology, 

 we may turn to the remaining groups of animals, the so-called In vertebrata, which, 

 as a whole, may be distinguished from Chordates merely by negative characters, 

 there being no pharyngeal slits, no notochord, and no central nervous system 

 running along the back. Nevertheless, some of the higher groups of invertebrated 

 animals — such as the Arthropods and Molluscs — resemble each other, and differ 



