xm PHYLUM CHORDATA 63 



size, perforated by very numerous gill-slits, and surrounded by an 

 atrium. The liver is a hollow pouch of the intestine. There is no 

 heart, and the blood is almost colourless. The nephridia remain dis- 

 tinct and open into the atrium. The brain is very imperfectly 

 differentiated ; there are only two pairs of cerebral nerves ; and the 

 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 coelome is an enterocoele. 



Affinities. Amphioxus has had a somewhat chequered zoo- 

 logical 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 

 (dorsal lamina) 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 metamor- 

 phosis. Of this, however, there is not sufficient evidence, and most 

 recent investigations 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, Keptiles, 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 



