1 64 INTRODDCnON TO ZOOLOGY. 



the lower extremity. On many occasions we have found 

 specimens of a small cnistacean* swimming about in the 

 branchial cavity, and looked upon it as a parasite, established 

 in its appropriate quarters, not as a casual occupant, destined, 

 like some unfortunate wight in the fairy tale, as food for the 

 Ogre into whose fortress it had intruded. 



But although some species of Ascidia are rough and darkish, 

 others of smaller size are possessed of glassy transparency, 

 and, when kept alive in vessels of sea-water, furnish a spec- 

 tacle of novelty and interest. Some of the compound species 

 are branched {Fig. 154); and such is their transparency, that 

 the movements of the internal organs can be distinctly seen. 

 This has enabled Milne Edwardsf to detect, in these animals, 

 a very singular condition of the circulating system. The 

 blood actually moves backwards and forwards, to and from 

 the heart, in the same vessel, which thus performs the office 

 both of a vein and of an artery, in the manner it was of old 

 supposed to do in the human body. The yoimg Ascidians 

 are not fixed to the place of their birth, but gifted for a short 

 period with locomotive powers, analogous to those of other 

 marine animals already mentioned. 



Some of these compound Ascidians are found arranged in 

 regular radiating patterns on the fronds of our large sea- 

 weeds. In such cases, the young, in its early state, has 

 possessed a reproductive power by gemmation or bads, analo- 

 gous to that of the larva of the medusa already mentioned 

 (page 37). This fact, which has been established by Milne 

 Edwards, explains the origin of the characteristic patterns 

 which they sometimes exhibit on rocks washed by the waves, 

 or on sea-weeds thrown upon the beach. These singularly- 

 formed creatures {Botrylli) are, in their colours, gay and 

 diversified, and their general aspect is such as would be 

 presented by minute but brilliant medusa, set with great 

 regularity round a common centre. 



Among the Tuuicata are some (Pyrosoma) which are found 

 in the open sea, especially in tropical climates, sometimes 

 united together in masses of more than a mile in extent, and 

 lighting up the sea by a beautiful pale gi-ecnish light, which 

 passes with great rapidity into the other prismatic colours. 



* Notodelphys ascidicola. For description and fignre, vid. Professor 

 Allnian, in Aiituils of Natural History, vol. xx. July 20, 1847. 

 f Sur les AsciJies composees des cotes de la Mauclie. 1841. 



