45 



for the highest powers, aud it shows that each of these ribs is 

 in fact a thin band of finely-striated mnscular fibres, which can 

 be traced all round the cage, aiid can be seen to divide into 

 smaller fibres and fibrills?, which split up and distribute them- 

 selves on the walls of the large tube at the side. We can 

 understand the uses of these muscles, which, by contracting, 

 can readily cause the walls of the cage to contract, and eject 

 water rapidly, as Ascidians are known to do, and from whicb 

 the larger ones get the popular name of sea-squirts. I think 

 it not improbable that the tube at the side also has muscular 

 fi'bres entering into its structure, but I have not yet been able 

 to demonstrate them. 



The end of the cage nearest to the body of the animal is 

 larger than the mouth, and joins by a tube the digestive cavity 

 of the Ascidian. 



If we turn again to the mouth of the animal we find seven 

 short leaf-like expansions, which probably admit of motion 

 during life (a, fig. 2), but cannot be extended and retracted 

 like the tentacles of the Polyzoa. We know that the absence 

 of vibratile 'tentacles in Ascidians led to great difference of 

 opinion between no less distinguished observers than Profes- 

 sors Allman and Huxley as to the exjict nature of the part I 

 have hitherto mentioned as the bird-cage structure. By Pro- 

 fessor Allman the true mouth of the Ascidian is supposed to be 

 at the farther end of the structure, near the digestive cavity ; 

 and the ribs and bars of the cage are believed to be the homo- 

 logues of the tentacles of the Polyzoa. By Huxley the end 

 nearest the outer membrane is believed to be the true mouth ; 

 and the cage-like structure is supposed to be a greatly modified 

 and dilated pharynx. My own observations have hitherto in- 

 clined me to the views of Professor Allman, but an attentive 

 study of the specimen now under notice leads to the belief 

 that Professor Huxley is more correct. I find attached just 

 within the cap a plentiful supply of conspicuous tentacles, 

 which are sufficiently long to be easily protruded when neces- 

 sary. In one of my mounts I have seen one or two of these 

 tentacles protruded through the mouth, but in the majoritv of 

 instances they are retracted, as in the Polyzoa, and lie curled up 

 within the cap. Pig. 2 is an enlarged view of the cap, and at 

 i are seen several of these tentacles as observed in a slide I 

 have here for exhibition. 



These cage-like bodies separate so easily from the other parts 

 of the Ascidian that hundreds of them were found floating in 

 the water under which the dissection was made. 



After the outer membrane had been removed as described, 

 there remained an apparenth" semi-solid substance in which 

 Ascidians in every stage of development were embedded. A 



