^28 ON THE CLASSES OF THE 



be confounded with the Acrita. It is only the other 

 day, indeed, that the last mentioned naturalist gave full 

 force and developement to discoveries of Gaertner and 

 Pallas, which had at first been neglected, and which at 

 last became totally forgotten. The true Polypes however, 

 as we have already seen, possess but one alimentary ca- 

 vity, which is provided with only one external aperture, 

 and is separated from the outer cuticle by a simple cellu- 

 lar substance. From these, then, the group we have now 

 under consideration differs, in having two apertures to their 

 intestinal canal, in being no longer a homogeneous mass 

 of pulp, but offering to the view two distinct tunics at 

 least, with a body divided internally into several cavities, 

 which are furnished with viscera. It is in these animals 

 we have the first indubitable vestige of a concentration of 

 the nervous system, and of organs indisputably constructed 

 for purposes of respiration, circulation, and generation. 

 But notwithstanding all these proofs of a more complicated 

 construction than that which the Acrita possess, some of 

 the Tunicata, such as the genus Eucalium, and that curi- 

 ous inhabitant of the Australasian seas, Sigillina Au- 

 stralis Sav., resemble the Polypi vaginati so much as to 

 require the eye of the most experienced anatomist to di- 

 stinguish them. 



The most singular however of the compound Tunicata 

 are those perhaps which, like the Botrylli, display se- 

 veral stelliform or radiant systems, disposed in circles, 

 ellipses, &c. round a central cavity or opening; the 

 whole appearing, at first sight, to be a thin transparent 

 radiated jelly coating marine substances. To a careless 

 observer this appearance might be sufficient to confound 

 them with the Radiata; but a little attention will prove to 



