TUNICATA. 7 



live entirely upon vegetable organisms. The contents of 

 the stomachs of the Phallusiac, Clavelinfe, and Diazonse 

 examined consisted of particles of florideoiis alga), which 

 had probably found their way there by chance, and a great 

 quantity of microscopic plants of low position in the series, 

 species of Navicula, Frustulia, Baccilaria, Closterium, &c. 

 These minute vegetable organisms have been shewn by 

 Nageli and Schmidt to contain cellulose. This is probably 

 dissolved by the gastric juice, that is to say, changed into 

 sugar or gum ; in which state it circulates with the blood, 

 and is afterwards introduced into the tunics, either directly 

 by the sanguiferous canals (as in Phallusia), or by their pro- 

 longations ramified in the walls of the common body (as in 

 Diazona and Botryllus), which thus, as Milne-Edwards has 

 shewn, contain also blood in their cavity, probably penetrating 

 by imbibition when the envelopes have no blood-vessels. The 

 presence of cellulose in the tunics of the Ascidian Mollusca, 

 then, cannot be taken as an evidence of an approach to a 

 vegetable nature in those bodies. Jt affords us, however, 

 a wholesome warning against the placing of confidence in 

 asserted chemical distinctions between the great kingdoms 

 of nature. 



If we consider our Bi-itish Ascidians in ascending order, 

 they will rank as follows; — 1st, those lower and com- 

 pound forms which constitute the " Ascidiens Composees" 

 of Milne-Edwards ; 2nd, a few species belonging to his 

 social group ; 3rd, a considerable number of simple Asci- 

 dians, properly so called ; 4th, a genus {Pelonaia) of 

 Tunicata, including as yet only two forms, which, on 

 account of important peculiarities of organisation to be 

 hereafter noticed, cannot take its place in any of Milne-Ed- 

 wards's sections, but must rank as the type of a special 

 section ; and, 5th, a solitary example of the great and very 

 numerous tribe of Salpidac. 



