xiii PHYLUM CHORDATA 65 



dorsal and ventral spinal nerves do not unite. There are no paired 

 eyes, but there is a median pigment-spot in the wall of the brain, and 

 many others in the spinal cord ; an auditory organ is absent. 'The 

 gonads are metamerically arranged and have no ducts. There is a 

 typical invaginate gastrula, and the mesoderm arises in the form 

 of metameric coelomic pouches. The ccelome is an enterocoele. 



Affinities. Amphioxus has had a somewhat chequered zoologi- 

 cal history. Its first discoverer placed it among-the Gastropoda, 

 considering it to be a Slug. When its vertebrate character was 

 made out, it was for a long time placed definitely among Fishes, as 

 the type of a distinct order of that class ; but it became obvious, 

 from a full consideration of the case, that an animal with neither 

 skull, brain, heart, auditory organs, nor paired eyes, with colourless 

 blood, with no kidneys in the ordinary sense of the word, and with 

 its pharynx surrounded by an atrium, was more widely separated 

 from the lowest Fish than the lowest Fish from a Bird or 

 Mammal. 



There was still, however, no suspicion of any connection 

 between Amphioxus and the Urochorda until the development 

 of both was- worked out, and it was shown that in many 

 fundamental points, notably in the formation of the nervous 

 system and the notochord, there was the closest resemblance 

 between the two. The likeness was further emphasised by the 

 presence in both forms of an endostyle, an epipharyngeal groove 

 and peripharyngeal bands, and of an atrium, and by the obvious 

 homology of the stigmata or gill-slits of Tunicates with those of 

 Amphioxus. The Urochorda being obviously a degenerate group, 

 it was suggested that the peculiarities of the adult Amphioxus 

 might also be due to a retrogressive metamorphosis. Of this, 

 however, there is not sufficient evidence, and all recent investiga- 

 tions have tended to bring the Acrania nearer to the Craniate 

 Vertebrata, and to remove them further from the lower Chordata. 



SECTION II. CRANIATA (VERTEBRATA). 



The group of the Craniata (Vertebrata) includes all those animals 

 known as Fishes, Amphibians, Reptiles, Birds, and Mammals, or, in 

 other words, Vertebrata having a skull, a highly complex brain, a 

 heart of three or four chambers, and red blood-corpuscles. 



In spite of the obvious and striking diversity of organisation 

 obtaining among Craniata between, for instance, a Lamprey, a 

 Pigeon, and a Dog there is a fundamental unity of plan running 

 through the whole group, both as to the general arrangement of 

 the various systems of organs and the structure of the organs them- 

 selves far greater than in any of the principal invertebrate groups. 



