TUNIC AT A. 275 



to the mouth, which indicates the oesophagus. About the same time 

 may be seen the outline of the anal orifice upon the internal integu- 

 ment : then the opaque yellow tunics of the dilated stomach, and tlie 

 reflected intestine appear ; and below these parts the pulsations of the 

 large transparent vasiform heart render that organ conspicuous. 



The whole of the viscera included by the smooth integument have 

 been observed to rotate in the cavity formed by the thick gelatinous 

 tunic, to which the visceral mass again becomes attached by the 

 adhesion of the muscular tunic at the branchial and anal orifices, and 

 by the establishment of corresponding orifices in the integument. 



Savigny was of opinion that the ovum of the compound Ascidian 

 contained the germs of all the individuals composing the characteristic 

 groups in the mature aggregate animal, and that their development 

 was simultaneous ; but the observations of Dr. Edwards have proved 

 that a second mode of reproduction, namely that by gemmation, is 

 superinduced upon the young Ascidian, after the foi'egoing develop- 

 ment from the impregnated ovum, which off'ers an interesting analogy 

 to the phenomena presented by the polype-larva of the Medusa 

 The individuals formed by the gemmation of the primary bud of 

 the young Ascidian, instead of being detached, are retained ; the 

 process of gemmation being regulated so as to produce the chai'ac- 

 teristic pattern in which the diflTerent individuals are grouped in the 

 mature compound animal. 



The gemmation commences by the development of a small tubercle 

 from the abdominal portion of the internal tunic of the young 

 Ascidian. This is prolonged, retaining an active circulation in its 

 interior, and is accompanied by a corresponding growth of the outer 

 gelatinous integument, which becomes clavate. The pi'ocess then 

 bifui'cates ; the divisions, in like manner, becoming elongated, ex- 

 panded and bifurcated at their extremities. Soon the outline of an 

 Ascidian is sketched in each of these extremities. The primitive 

 connection with the jiarent is obliterated ; but the young individuals 

 remain united together by their primitive peduncle, according to 

 the law which determines their mode of grouping into systems. 

 By the progressive increase of their outer gelatinous integument, they 

 finally coalesce and form the compound mass. 



The second order of the class Tunicata includes the Salpians, 

 which float in the open sea, and are chai'acterised by their transparent 

 elastic outer tunic, which is elongated, compressed, and open at both 

 extremities. The muscular fibres of the mantle, or membrane lining 

 the cartilaginous tunic, are arranged in flattened bands. The moutli 

 and stomach, the liver and the heart, are aggregated in a small mass 

 or nucleus, near the anterior aperture of the tunic ; the intestine ex- 



T 2 



