272 



LECTURE XX. 



r^' 



In consequence of the space between this and the outer tunic being 

 closed, that tunic accompanies the muscular tunic in its contraction, 

 through the influence of the surrounding pressure ; when the muscle 

 ceases to act, the elasticity of the outer coat begins to restore the 

 fibrous sac to its former capacity, and the surrounding water flows 

 into its cavity, either directly or by distending the branchial sac. 

 We shall find other instances of the economising of muscular force 

 by the substitution of elasticity as we ascend in the survey of the 

 molluscous organisation. 



In the small compound Ascidians the organisation is essentially 

 like that of the solitary species, but the viscera are somewhat dif- 

 ferently disposed : the cavity is longer and narrower, the entire 

 animal viewed singly being more vermiform. 



In Jig. 116., from Dr. M. Edwards' elaborate memoir, the anatomy 

 of one of the individuals of the species which he has 

 called Amarouciuin proliferum, extracted from the 

 common investing tunic, is displayed, a is the proper 

 or muscular tunic, in which most of the fibres are 

 longitudinal ; it is much more feeble than in the 

 solitary Ascidians : c is the oral or branchial orifice : 

 e, the branchial sac : z, the anal or cloacal outlet ; it is 

 protected by the overhanging valve 'i, which is re- 

 quired by the compound Ascidians on account of the 

 excretory outlet in the muscular tunic communicating 

 with a common cloacal cavity in the external tunic, 

 around which the individuals of the composite series 

 are grouped : j is the ganglion of the nervous system : 

 k, the short and wide oesophagus : /, the stomach, the 

 exterior of which is rendered shaggy by the appended 

 biliary follicles ; m, is the intestine : 7i, the anus : o, 

 is the heart, which, by its remoteness from the 

 branchial sac, diff'ers more in relative position from 

 its analogue in the simple Ascidians than any other 

 viscus ; it is provided with a pericardium o i p is the 

 ovarium, p^' an ovum about to escape through the 

 cloacal outlet, with the embryo ripe for exclusion. 

 The most important structural difference between the 

 aggregate and solitary Ascidians is the combination 

 in the former of a male apparatus with the ovarium. 

 In Jig. 115. g is the testis : r, the vas deferens, which 

 terminates at r\ in the common cavity of the muscular tunic. 



Some of the compound Ascidians are ramified, and their tunics so 

 transparent as to permit the movements of the internal organs to be 



m 



Compound Ascidian. 



