914 POLYZOA AND TUNIC ATA 



drawn in through the mouth by the action of the cilia of the bran- 

 chial sac and of the alimentary canal ; a part of this current passes 

 through the fissures of the branchial sac into the peribranchial 

 cavity, and thence into the cloaca ; whilst another portion, entering 

 the stomach by an aperture at the bottom of the pharyiigeal sac, 

 passes through the alimentary canal, giving up any nutritive 

 materials it may contain, and carrying away with it any excre- 

 mentitious matter to be discharged ; and this having met the 

 respiratory current in the cloaca, the two mingled currents pass forth 

 together by the atrial orifice, i. The long post-abdomen is principally 

 occupied by the large ovarium, p, which contains ova in various stages 

 of development. These, when matured and set free, find their way 

 into the cloaca, where two large ova are seen (one marked p and 

 the other immediately below it) waiting for expulsion. In this posi- 

 tion they receive the fertilising material from the testis, q. which 

 discharges its products by the long spermatic canal, r, that opens into 

 the cloaca, r . At the very bottom of the post -abdomen we find the 

 heart, o, inclosed in its pericardium, o' '. In the group we are now 

 considering a number of such animals are imbedded together in a 

 sort of gelatinous mass, and covered with an integument common to 

 them all ; the composition of this gelatinous substance is remarkable 

 as including cellulose, which generally ranks as a vegetable product. 

 The mode in which new individuals are developed in this mass is by 

 the extension of stolons or creeping stems from the bases of those 

 previously existing ; and from each of these stolons several buds may 

 be put forth, every one of which may evolve itself into the likeness 

 of the stock from which it proceeded, and may . in its turn increase 

 and multiply after the same fashion. 



In the family of Didemnians the post-abdomen is absent, the heart 

 and generative apparatus being placed by the side of the intestine in the 

 abdominal portion of the body. The zooids are frequently arranged 

 in star-shaped clusters, their anal orifices being all directed towards 

 a common vent which occupies the centre. This shortening is still 

 more remarkable, however, in the family of Botryllians, whose 

 beautiful stellate gelatinous incrustations are extremely common upon 

 seaweeds and submerged rocks (fig. 691). The anatomy of these 

 animals is very similar to that of the Amaroucium already described ; 

 with this exception, that the body exhibits no distinction of cavities, 

 all the organs being brought together in one, which must be con- 

 sidered as thoracic. In this respect there is an evident approximation 

 towards the solitary species. 1 



This approximation is still closer, however, in the ' social ' Asci- 

 dians, or Clavellinidce, in which the general plan of structure is 

 nearly the same, but the zooids are simply connected by their stolons 

 instead of being included in a common investment ; so that their 

 relation to each other is very nearly the same as that of the poly- 



1 For more special information respecting the compound Ascidians see espe- 

 cially the admirable monograph of Professor Milne-Edwards on that group ; Mr. Lister's 

 memoir, ' On the Structure and Functions of Tubular and Cellular Polypi, and of 

 Ascidise,' in the Phil. Trans. 1834 ; and the article ' Tunicata,' by Professor T. Rupert 

 Jones, in the Cyclopcedia of Anatomy and Physiology. More recent authorities 

 are cited on p. 918. 



