TUNICATES 167 



ear-like organ; and the remarkable fact is that the retina and lens 

 of the eye and the sensory part of the "ear" are derived from the 

 walls iif the brain as is the case in vertebrates. 



After swimming about for some time the tunicate tadpole usu- 

 ally settles down upon the bottom and fastens itself to a situation 

 from which it never departs. Under these conditions it finds 

 almost no need for sense organs or skeleton, and accordingly these 

 degenerate to such a degree that the eye disappears completely, 

 and the other structures are reduced to mere remnants, 



f?ut all tunicates do not thus settle down and degenerate, for 

 some of them remain active throughout life, swimming rapidly 

 tlirough the water. One of these free forms called Appendieidatia 

 remains tadpole-like in shape throughout its existence, having a 

 pair of gill-slits, one on each side, and a long powerful tail which is 

 provided with a fin, and arises from the middle of the ventral side 

 of the body. 



Many tunicates are solitary animals while others produce 

 large colonies by budding, the older members of the colony giving 

 rise to the j'oitnger. In other forms such as Salpa, there is an alter- 

 nation of generations, one being produced from eggs and the fol- 

 lowing generation through budding. 



A tunicate has been aptly compared to a leather bottle with 

 two spouts. The outer covering of the body is usually tough 

 in consistenc}' and contains cellulose, the composition of which is 

 identical with the substance that forms the avails of plant cells. 



A moment's observation of the two funnel-shaped spouts will 

 show that water is constantlj' being drawn into one and forced out 

 from the other, and a further study shcnvs that the water is drawn 

 in at the spout at the front end of the body, and passed out of the 

 opening upon the back of the animal. The intestine is U-shaped, 

 and the mouth is at the place where the water enters while the vent 

 is at the spout through which the water passes out. 



The throat is a wide sac almost as long as the body itself, and 

 is pierced by so many little gill slits that its sides resemble a sieve. 

 The water enters the mouth, passes through these gill slits, and 

 finally out through the dorsal spout; the current being maintained 

 by the beating in unison of thousands of hair-like cilia which line 

 the gill slits. 



