Summary. 77 



the mouth. The heart (o) lies at the lowest part of the 

 body and from it the vessels (<?') pass into the wall of the 

 branchial chamber. In the action of this heart a curious 

 appearance is observed ; the blood is driven by this 

 vessel first from one end to the other, for a second the 

 action stops, then it is resumed in the opposite direc- 

 tion, again another cessation, and another reversal, &c. 



The ' tunic ' or outer wall (a) contains a cellulose- 

 like compound which is interesting as it is almost the 

 only instance of the occurrence of a carbo-hydrate of 

 this kind in the Animal Kingdom. 



Young tunicates as they emerge from the egg 

 appear as small, tailed larvos, with bodies consisting 

 of two cavities. The axis of the tail consists of a 

 cartilaginous or gristly rod ; in one cavity of the body 

 the nerve ganglion is developed, in the other space the 

 viscera are formed. Thus they foreshadow the struc- 

 ture of vertebrate animals, and on this account some 

 zoologists raise this class to the rank of a sub-kingdom. 



Tunicaries are sometimes solitary, but many species 

 are found united into social assemblages, and this 

 union may go as far as the perfect union of the 

 blood-vessel systems, a single vascular apparatus sup- 

 plying the whole colony. In one group, the Salpae, 

 there is an alternation of generations, solitary and 

 colonial forms succeeding each other in a cycle. 



Many of the tunicates are phosphorescent, Pyro- 

 soma, a compound form inhabiting the Atlantic Ocean, 

 being the most luminous of marine animals. 



Summary. The chief types of worms may be 

 tabulated thus : 



i. Unjointed, ciliated, non-parasitic forms without 

 ciliated head-lobes -= Class Turbellaria. 



