EXTERNAL FORM. 1 7 



In young transparent individuals, such as that of which 

 the anterior portion is represented in Fig. 3, the pharynx, 

 or first division of the digestive tract, into which the 

 mouth leads directly, can be seen through the body-wall, 

 and it is found to be perforated on each side by a great 

 number of elongated vertical slits, whose number varies 

 with the age of the individual, but may eventually attain 

 the astonishing figure of 180 pairs. They are the gill- 

 clefts opening from the pharynx into the atrial chamber. 

 In the living Amphioxus an almost continuous stream of 

 water is being drawn through the mouth into the pharynx 

 for purposes of respiration and nourishment, then pass- 

 ing out of the pharynx, by way of the gill-clefts, into 

 the atrial chamber and thence to the exterior through the 

 atriopore. 



Craniiim and Scusc-organs. 



Besides lacking differentiated lateral fins, Amphioxus 

 differs fundamentally from the higher Vertebrates in the 

 absence of a craniuvi, of paired eyes, and paired or un- 

 paired auditory organs. 



On account of the absence of a cartilaginous cranium 

 it has been placed by itself in a separate division, the 

 Acrania, in contrast to all the other Vertebrates proper, 

 from the Cyclostomata upwards, which all possess a 

 cranium of one sort or another and are hence known as 

 the craniate Vertebrates or Crafiiota. In Amphioxus the 

 only cartilage in the head-region consists of a ring lying 

 round the margin of the oral hood at the base of the 

 buccal cirri. It is formed of separate pieces correspond- 

 ing to the number of the cirri, and each piece sends up a 

 process into its adjacent cirrus, so that the latter is pro- 

 vided with a stiff skeletal axis (Figs. 3 and 4). These are 



