508 PHILOSOPHY OF ZOOLOGY. 



Section II. ACEPHALA TUNCATA. 



Covering soft or corriaceous. 



The formation of this interesting group of animals was 

 first publicly announced by LAMARCK in his Histoire Na~ 

 turelle des Animaux sans Vertebres, Tom. iii. p. 80. (1816). 

 The labours of DESMARET, LESUEUR, and CUVIER, aided 

 by the descriptions of ELLIS and PALLAS, paved the way 

 for the masterly efforts of SAVIGNY, to whom we owe the 

 most extensive, new, and accurate information yet given, 

 concerning the animals of this group. His observations are 

 contained in his RecJierches Anatomiques sur les Ascidies 

 composees, et sur les Ascidies simples, inserted in his Me- 

 moires sur les Animaux sans Vertebres. Svo. Paris, 181 6. 



The covering of the animals of this group consists of 

 an external and internal sac or tunic, either entirely united, 

 or unconnected, except at the apertures. The surface is 

 smooth in some, and rough in others, and in a few spe- 

 cies defended by an artificial covering of agglutinated shells 

 and sand. The sacs are furnished with muscular bands, 

 and are capable of contraction. Some of the species, by 

 means of contractile movements, float about in the water ; 

 others, receiving that element into the branchial cavity, and 

 ejecting it forcibly at the opposite one, push themselves for- 

 ward. Many, however, are fixed during life to sea-weeds 

 and stones. 



The apertures of the tunic are two in number, The one, 

 frequently the largest, is destined for receiving the water 

 into the cavity, to supply the mouth and gills. This is 

 termed the branchial cavity. The other is destined for the 

 exit of the water, the eggs, and the foeces, and termed the 

 anal opening. These apertures are sometimes placed near 

 each other, at other times at opposite extremities of the bo- 

 dy, and variously provided with tentacula or valves. 



