VERTEBEATA. 



309 



Fio. 280.— Au Ascidian. 



Division I. — Urochordata. 

 Class I. — Tunicata. 

 The Tunicates form a small and singular group of animals 

 having relations with the worms on the one hand and with 

 the Vertebrates on the other. The most common forms 

 (the solitary Ascidians) are 

 enclosed in a leathery, elastic 

 bag, one end of which is fast- 

 ened to the rocks, while the 

 other has two orifices, for the 

 inlet and exit of a current of 

 water for nutrition and res- 

 piration. They are without 

 head, feet, arms, or shell. In- 

 deed, few animals seem more 

 helpless and apathetic than these apparently shapeless be- 

 ings. The tubular heart exhibits the curious phenomenon 

 of reversing its action at brief intervals, so that the blood 

 oscillates backward and forward in the 

 same vessels. Another peculiarity is the 

 presence of cellulose in the skin. The 

 water is drawn by cilia into a branchial 

 sac, an enlargement of the first part of 

 the intestine, whence it escapes through 

 openings in the sides, to the exeurrent ori- 

 fice, while the particles of food drawn in 

 with the water are retained and passed 

 into the intestine. The larva is active, 

 swimming by means of a long tail. It 

 looks like a tadpole, and has a notochord 

 and a nervous system closely resembling 

 Fi«.28i.-Diagramorsim. ^'lose of a Vertebrate. Afterwards it at- 

 pieAscidian: B,5,i)ran- tachcs itself by the head, the tail is ab- 



chial sac ; n, nervous i i i i 



ganglion; 8, stomach; <, SOrbed, and the nervous system is re- 

 in testlne; 0, renroduc- j ■, , ., ^■^ ^• rni 



live organ ; ft, iieart. duced to a Single Small ganglion. Thus 



