CHORD AT A: VERTEBRATA. 335 



as to its nature in Vertebrata. It is either a stomodaeum the view taken later 

 on, as there is certainly an ectodermic invagination or it represents a modified 

 pair of gill-clefts. In favour of this last-mentioned view, it is urged (i) that the 

 fifth nerve branches over the dorsal angle of the mouth, just as e. g. the glosso- 

 pharyngeal nerve does over the first branchial cleft ; (2) that a branchial sense-organ 

 lies at each dorsal angle from which the Gasserian ganglion is derived like the ganglia 

 of the other segmental cranial nerves ; (3) that in some Teleostei it appears as a 

 double pit. 



The part of the archenteron which occupies the tail in the larval Urochorda 

 is metamorphosed partly into muscle-cells, partly into blood corpuscles. There is 

 generally said to be a post-anal section of the archenteron in Vertebrata which com- 

 municates by a neur-enteric canal with the neural tube. But it is possible that 

 the neural and archenteric tubes both open at the blastopore, as in certain Am- 

 phibia (Rana, Alytes, and Triton], where the blastopore certainly persists as anus ; 

 and that as the tail grows out, the blastopore is gradually shifted to the ventral 

 surface, the neural tube still retaining its connection with it *. This view ascribes 

 a neural origin to the post-anal gut, and appears to explain its subsequent disappear- 

 ance. See Spencer, Q. J. M. xxv : cf. Durham : Miss Johnson : Q. J. M. xxvi. 



For the Hemichordata of Bateson, see Enteropneusta in the Vermes. 



SUB-PHYLUM VERTEBRATA. 



CHORDATA, which possess the following characters. The integument 

 is composed of an epidermis consisting of several layers of cells, and a dermis 

 chiefly composed of fibrous connective tissue. There is a well-developed endo- 

 skeleton, consisting of a cranium with move able jaws : a backbone with neural 

 and sometimes haemal, arches, and vertebral centra replacing the notochord 

 more or less completely : and two pairs of limbs, pectoral and pelvic. The 

 nerve-cord is enlarged anteriorly, forming the brain which is lodged within 

 a cranium. There are two olfactory pits and two eyes. The lens of the eye 

 is cellular, and developed from epiblast : the retina is cup-shaped, and derived 

 from a vesicle of the brain. A contractile fibro-muscular iris, placed in front 

 of the lens, controls the amount of light entering the eye. There are also two 

 auditory organs arising as involutions of the epiblast which close and usually 

 become highly complicated. A ventrally placed section near the head of the 

 closed blood-vascular system is differentiated into a rhythmically contractile 

 heart. There is a set of haemoglobin-containing blood-corpuscles or 

 haematids, with fixed outlines in addition to the amoeboid corpuscles, or leuco- 

 cytes. There is a blood-making organ the spleen lodged in the meso- 

 gastrium and traversed by the blood-current. And there is a system of 

 lymphatic channels communicating with the blood-vascular system. The 

 coelome is large and lodges the heart, the main portion of the alimentary 



- 



1 In the Cyclostome Petromyzon the post-anal gut is represented by a solid rod of cells. The 

 blastopore persists as the anus ; it is, at first, dorsal, but shifts to the ventral surface in consequence 

 ' of the elongation of the embryo and the formation of the tail.' Shipley, P. R. S. xxxix. 1885. 



