236 AN INTRODUCTION TO ZOOLOGY. 



anus, and the generative products whicli are also sent 

 into it by their efferent ducts. The heart undergoes 

 regular contractions, but they sometimes reverse their 

 direction, thus reversing the blood-stream, a very 

 curious fact. The animals may be either fixed or 

 floating, single or colonial. They sometimes present 







A 



rig. 86.— Compound Ascidian (Boti-ylius), one of the Ascidiaoeae. 0, moutli 

 of one individual. A, excurrent aperture common to the group of individuals. 



an alternation of generations. The Ascidians are a 

 group in which the larval form differs considerably 

 from the adult. The larva in some cases possesses 

 eyes which are lost in the adult. The tail, furnished 

 with a urochord, which is usually atrophied in the 

 adult, has been already named (p. 77). 



