90 



PROF. AGASSIZ'S 



around, and a structure of the alimentary canal 

 similar to that of the other Bryozoa. This Pedi- 

 cellina shows that among the so called Infusorial 

 animals, the Vorticelloe, placed among those which 

 have been up to this time considered as a natural 

 group, should be separated from them. In my 

 opinion, Vorticellseare closely allied to the type of 

 Bryozoa, living separately, and constitute a fresh 

 water genus of the type Bryozoa. 



Among these Bryozoa, (Plate XLVIII,) there are 

 some which are very remarkable for the curious 

 appendages which surround the main parts of their 

 body. Generally, there are some large cells, and 

 around them smaller ones, (Plate XLVIII, fig. J,) 

 independent buds, as itwere,with threads, (Fig.F,) 

 or with articulated joints, which shut and open 

 like the beak of a bird, (Fig. B). What these are 

 is scarcely understood, and I shall hardly venture 

 to express my opinion about them. 



Buds which rise from a common stem, and which 

 differ from other buds, we have observed among 

 MedusjB , even buds which perform different 

 functions from others. And I can scarcely help 

 thinking, that in these Bryozoa there are buds 

 formed upon the same stem which will not grow 

 in the same manner as the main individuals, but 

 assume an entirely different shape, and will be 

 '* catching individuals,'' living to supply the stom- 

 ach with food by seizing upon little animals, and in- 

 troducing them into the cavities of the main body. 

 To consider those appendages as parts of the main 

 animal is out of the question, (Plate XLVIII, fig. 

 C), as they have no true connection with them. 

 To consider them simply as peculiar appendages 

 to those animals, would not be more rational. But 

 when we see that these Bryozoa can form stems so 

 complicated, or rather containing so many indi- 

 viduals, I do not see why we should not recognize 

 imperfect individuals, like buds, assuming a 

 peculiar form of their appendages, and then ad- 

 mit that they are analogous to the compound in- 

 dividuals of Medu?ie. in which the isolated indi- 

 viduals do not perform the same functions, and 

 do not resemble strictly each other. If this view 

 be correct, I would also venture to hint at the 

 probability of Pedicellaria in Echinoderms being a 

 kind of budding of very imperfect individuals, re- 

 sembling the lowest forms of the class, the Crl- 

 noids, and living as a sort of low parasites upon 

 the parent, from which they differ more than any 

 other kind of buds. Thus exemplifying those 

 higher beings to which all sorts of parasites attach 

 themselves constantly. 



Ascidise have a mode of development which re 

 sembles in its modifications, somewhat, the Bry- 

 ozoa, but in other respects differs. The eggs of 

 Ascidise, which have been observed, are free when 

 laid, and from them arise free germs. You may 

 trace in Plate XLIX, all the phases of the devel- 

 opment: first, the development of the yolk into 

 spheres, which grow to form a uniform germ, (Fig. 

 D) which divides by a deep depression (Fig. C) into 



[PLATE XLIX. DEVELOPMENT OP 



an anterior and a posterior mass; the anterior be- 

 ing transformed into a sort of head (Plate XLIX, 

 fig. A ). When hatched they resemble very much 

 Tadpoles, and they move like Infusoria, or rather 

 like Cercarias. Next appendages are developed or 

 the two sides of the animal (Fig. E ). We see that 

 here, even in these compound Ascidiaus, the bi- 

 lateral symmetry of the parts is still characteristic. 

 There are no great modifications or changes in any 

 of them after they have grown to a certain size. 

 This external coating, which is not an egg-shell, 

 gradually enlarges and separates more extensively 

 from the germ itself, and finally is transformed 

 into a shell like, or rather a membranous envelope, 

 like this (Plate XLVII, fig. H),and the germ is trans- 

 formed into an Ascidiaproper,with all the structure 

 which characterizes the perfect state of those ani- 

 mals there being those two openings to the exter- 

 nal covering (Plate XLIX, fig. J),and with those the 

 external masses of the animal proper; so that 

 these Ascidise closely resemble the Bryozoa, and 

 we pass at once from them to the bivalves. How 

 the compound Ascidise are developed, is not fully 

 known, though their embryology has been traced 

 in some instances, (Plate XLVII, fig. B.) Facts of 

 great importance have yet to be ascertained, 

 and I do not suppose that the facts which have 

 been studied on the growth of compound Ascid- 

 ians are completely understood, except in one 

 class in the Salpa in which a wonderful alter- 

 nation of generations has been observed. We have 

 here (Plate L), those curious animals known under 

 the name of Salpa, a kind of Acephala, which are 



