TUNICATES OR ASCIDIANS AND LANCELETS II 5 



water with which they are distended through an opening 

 by a sudden muscular contraction. 



If a specimen of the large Flask Ascidian (Ciona) be 

 examined, it will soon be apparent that the animal has 

 little in common with a sponge, sea anemone or mollusc. 

 The inner body wall is pierced with innumerable slits that 

 form a sieve which serves not only for the purpose of respira- 

 tion, but as a strainer that catches the minute organisms 

 upon which the Ascidian feeds. Water is continually 

 taken in through one funnel-like opening and, when robbed 

 of its food content and oxygen, is ejected through another 

 close beside it. 



The creature has a well-defined heart, liver, stomach 

 and intestine. How nearly akin to higher animals these 

 inert creatures actually are, however, was not suspected 

 until a minute tadpole-like animal was discovered to be 

 the larva of this living sac. This larva in many ways 

 offers a striking parallel to the tadpole form of frogs and 

 newts. It has a definite eye, brain, ear and mouth, 

 digestive track, heart, a nervous system, and above all 

 a notochord, similar to that formed in the development 

 of every backboned animal. For a period, varying accord- 

 ing to the species, the larva, hatched from an egg, leads 

 a free and independent life, swimming vigorously by means 

 of a long tail fin. There comes, however, a time when it 

 attaches itself to some object— weed, rock or harbour 

 pile, and then undergoes a strange course of degeneration, 

 forfeiting its notochord, tail, part of its nervous system, 

 and indeed everything save the few organs essential to 

 existence. The final form may vary to an extraordinary 

 degree, both in shape and ways of life. 



