58 ZOOLOGY SECT. 



Distinctive Characters. --The Acrania may be defined as 

 Vertebrata in which the note-chord extends to the anterior end of 

 the snout, in advance of the central nervous system. There is no 

 skull, and no trace of limbs. The ectoderm consists of a single 

 layer of cells which may be ciliated. The pharynx is of immense 

 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 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 : the 

 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 enteroccele. 



Affinities. Amphioxushas 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 real suspicion of " invertebrate ' 

 affinities until the development both of Amphioxus and the 

 Urochorda 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 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 no evidence, and all recent investigations and 

 especially the discovery of the nephridia, have tended to bring the 

 Acrania nearer to the Craniate Vertebrata, and to remove them 

 urther from the lower Chordata. 



DIVISION B. CRANIATA. 



The group of Craniate Vertebrata includes all those animals 

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



