166 THE MICROSCOPE AND ITS REVELATIONS. 



seen the oesophagus, which is continuous with the lower part of the pha- 

 ryngeal cavity; this leads to the stomach /, which is surrounded by biliary 

 follicles; and from this passes-off the intestine m, which terminates at n 

 in the cloaca, or common vent. A current of water is continually drawn- 

 in through the mouth by the action of the cilia of the branchial sac and 

 of the alimentary canal; a part of this current passes through the fissures 

 of the branchial sac into the thoracic sinus, and thence into the cloaca; 

 whilst another portion, entering the stomach by an aperture at the bottom 

 of the pharyngeal 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 anal orifice i. The long post-abdomen is principally occupied by the 

 large ovarium, p, which contain 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 position they receive the fertilizing 

 influence 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, 0'. 

 In the group we are now considering, a number of such animals are im- 

 bedded together in a sort of gelatinous mass, and covered with an integu- 

 ment 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. A communication between the circulating sys- 

 tems of the different individuals is kept-up, through their connecting 

 stems, during the whole of life; and thus their relationship to each other 

 is somewhat like that of the several polypes on the polypidom of a Cam- 

 panularia (519). 



557. 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 ar- 

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

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

 more remarkable, however, in the family of Boctryllians, whose beautiful 

 stellate gelatinous incrustations are extremely common upon Sea-weeds 

 and submerged rocks (Fig. 383). The anatomy of these animals is very 

 similar to that of the Amoroudum already described; with this exception, 

 that the body exhibits no distinction of cavities, all the organs being 

 brought together in one, which must be considered as thoracic. In this 

 respect there is an evident approximation towards the solitary species. 1 



558. This approximation is still closer, however, in the ' social 9 

 Ascidians, or Clavellinidce; in which the general plan of structure is 

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



1 For more special information respecting the Compound Ascidians, see espe- 

 cially the admirable Monograph of Prof. Milne-Edwards on that group; Mr. 

 Lister's Memoir ' On the Structure and Functions of Tubular and Cellular Polypi, 

 and of Ascidiae,' in the " Philos. Transact.," 1834; and the Art. Tunicata, by Prof. 

 T. Rupert Jones, in the " Cyclopaedia of Anatomy and Physiology." 



