TUNICATA. 



305 



FIG. 278. An Ascidian. 



Subkingdom VII. TUNICATA. 



This small and singular group of animals has relations 

 with the worms on the one hand and with the Vertebrates 

 on the other. The most common forms (the solitary As- 

 tidians) are enclosed in a 

 leathery, elastic bag, one end 

 of which is fastened 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 excurrent 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 

 Fm.279.-Diagramorsim- those of a Vertebrate. Afterwards it at- 

 pieAscidian.- fl,s,bran- taches itself by the head, the tail is ab- 



chial Bac ; n, nervous 



ganglion ; , stomach ; f, sorbed, and the nervous system is re- 



iutestine ; o, reproduc- i t " . -i. 



tive organ; A, heart, duced to a single small ganglion. 

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