152 



Descriptive Zoology. 



posterior end of the pharynx the intestine continues to the 

 anus, situated posteriorly and ventrally. By the action oi 

 the cilia water is taken into the mouth, passes through th< 

 slits in the wall of the pharynx, and enters a space around 

 the pharynx, called the atrial or peribranchial chamber, 

 whence it escapes to the exterior through a ventral opening 

 called the atrial pore. The lancelet usually lies buried 

 in the sand, with only the mouth projecting. It gets both 

 food and oxygen from the water, which is circulated 

 through the body by means of ciliary action. The lancek 

 occasionally swims by fishlike movements. 



Classification of the Lancelet. It might, at first thought, 

 seem strange that so simple an animal should be class 

 with a group having such complex structure as the vert* 

 brates. The lancelet has, in fact, been placed with the 

 mollusks, and later with the fishes, but is now located at 

 the foot of the vertebrate series, chiefly on account of th< 

 possession of the notochord and the dorsal nervous system. 

 It is really hard to locate an animal with colorless blood, 

 and with neither skull, brain, heart, auditory organs, paired 

 eyes, nor paired fins. 



The student who gets his ideas of classification almost 

 entirely from reading is apt to think that the animal king- 

 dom is divided into groups separated by clear and distinct 

 dividing lines. But when he undertakes the actual exami- 

 nation of any considerable series of animals, he often find; 

 that two groups, which he regarded as distinct, actuall] 

 merge one into the other so gradually that he finds it diffi- 

 cult to see just where the line of division should be drawn. 

 The line of demarcation must frequently be so drawn that 

 it cuts across some intermediate forms, part of whose char- 

 acteristics lie on one side and part on the other. In som< 

 cases the intermediate forms are living ; in other cases thi 



