VERTEBRATA. 241 



the mouthj is squeezed through the gill-slits into a 

 cavity, corresponding with the atrial cavity of the 

 Ascidian, which opens to the outside by an atrial 

 pore. In the typical Ascidians, the atrial pore is also 

 the excretory opening; but in the Ampldoxus there is 

 an anus as well, situated a little more towards the tail. 

 Structures which appear to be rudimentary fins, in 

 the form of long, but boneless, ridges suggest that 

 Amphioxus resembles the true fishes; so does also 

 the disposition of its muscles, which form, as in fishes, 

 regular successive masses. These successive masses 

 of muscles have been considered to be signs that the 

 Vertebrates are descended from animals that were 

 metameric like the worm or lobster. 



In other respects, however, the structure of Amphi- 

 oxus is very low ; not only has it no true head, but its 

 eye is only represented by an unpaired pigment spot, 

 its nose is represented by a pit lined with ciliated 

 cells, and it has no ears at all. Instead of a definite 

 heart, it has pulsating blood-vessels. 



The young Amphioxus differs from the adult suffi- 

 ciently to be spoken of as a larval form. It com- 

 mences its existence as a ciliated gastrula. 



The true Vertebrates. — The complexity of the 

 organs of the Vertebrates is so great that any sys- 

 tematic description of them would be impossible in a 

 small book like this. We can therefore only just 

 touch on a few of the salient points of the chief 

 groups, leaving the reader to consult the larger text- 

 books referred to (pp. 323-4) for further information. 



