476 



LECTURE XX. 



petals of a compound flower from a common base ; others, as the 

 Botryllus, being arranged in circles round a common central aper- 

 ture, beneath which the anal extremity of the intestine of each 

 individual terminates; whilst many of these circles of individuals 

 are aggregated together, and enveloped in a common cellulose tunic. 

 The substance of this is. in some species, e. g. Leptodi7ium, crowded 

 with calcareous granules ; m oih^vs^'Q. ^. Botryllus, it exhibits dis- 

 tinct fibres. 



In fig. 180., from Milne Edwards' elaborate memoir*, the anatomy 

 of one of the individuals of the species which he has igo ^^^ 

 called Amaroucium 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 so- 

 litary Ascidians : c is the oral or branchial orifice ; 

 e,f, the branchial sac ; e, 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 ; n, the anus ; 

 o is the heart, which, by its remoteness from the 

 branchial sac, diff"ers more in relative position from 

 its homologue in the simple Ascidians than any other 

 viscus ; it is provided with a pericardium 6 : 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 ^'"P""" 

 fig. 180, g is the testis, and r, the vas deferens, which terminates at 

 r\ in the common cavity of the muscular tunic. In Botryllus the 

 branchial sac resembles, in its relative size and position to the intes- 

 tine, that of most solitary Ascidians. 



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

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

 studied in the living animal. A very singular condition of the circu- 

 lating system has thus been detected.f The blood actually moves 



* CCXCIV. t CVII. p. 379. 



