92 



PROF. AGASSIZ S 



But remarkable as it is, there are no metamor- 

 phoses observed in these animals. Very early in 

 the young,the form of the adult is fully developed. 

 Here is a young Salpa (Plate L, fig. B) developed 

 within a compound chain, in which you observe 

 all the principal organs as they are in the perfect 

 animal the muscular fibres below, as they are 

 observed here, (Fig. C) the gill as it is also seen, 

 the heart as it is observed in (Fig. B). The appa- 

 ratus of the liver and alimentary canal, (Fig. B) 

 separated here, but combined here (Fig. C) in one 

 mass. 



Now the phenomena of alternate generation, of 

 which I have spoken, in the class of Worms, is 

 more complicated than in the Mollusca, as in the 

 Worms we have not only alternations in the gen- 

 eration, but also metamorphoses in each genera- 

 tion ; this opens a field of investigation, which 

 will present endless difficulties and endless details 

 to ascertain, but which will certainly go to enlarge 

 our views of animal structures and of individual 

 life. 



The embryology of the Bivalve Shell has not 

 yet been traced to that extent to which other classes 

 have been traced. It is remarkable that, though 

 they are so common though we have fresh wa- 

 ter bivalves though we have so many marine 

 bivalves, and some of them so exceedingly abun- 

 dant, their development has not been traced with 

 any degree of precision. The growth of the Oyster, 

 which might be traced every where, has never been 

 watched by any one. Even the Muscles are very 

 imperfectly known. Prof. Carus has observed the 

 fact, that from a very early period, the germ (Plate 

 LI, fig. A) of Anodonta has a tendency to divide 

 on one side, and the other side to flatten ; so that 

 the animal assumes an oblong shape with a disc 

 covering it from above. 



[PLATE LT. GERMS OF ANODONTA ] 



Professor Beneden says he has ascertained that 

 those germs have been mistaken for Infusoria, and 

 that the Leucophrys Anoclonta of Ehrenberg is 

 only a germ of a fresh water Clam. So that we 

 would thus have another evidence of the hetero- 

 geneous nature of that class of Infusoria. Perhaps 

 we should not insist so strongly upon these mis- 

 takes, when we remember how much Ehrenberg 

 has done to illustrate the lower animals. That 

 mistakes must have occurred constantly when the 

 metamorphoses of the more perfect animals were 

 less understood, is very natural. A peculiarity of 

 the Bivalves, in their growth, consists in the fact 

 that even those which have a foot developed as 

 a large fleshy mass between their two valves, have, 

 when young, only a small transverse bundle of 



fibres uniting the two valves, (Plate LI, fig. D) 

 and throw out a kind of byssus, which we observe 

 between the muscle, in Plate LI, fig. D. This 

 fact is important, as it shows that the shells which 

 have a byssus above the foot, should be considered 

 as lower than those in which the foot is more 

 largely developed, and can be expanded and con- 

 tracted between the two shells. 



Lastly I would mention the changes which Gas- 

 teropoda or snail-like animals undergo. During 

 their growth they have been traced in several 

 types. The changes of snails were early observed, 

 more recently the metamorphoses of naked Mol- 

 lusca. As they have very recently been more ful- 

 ly investigated, (Plate XLVIII) I would rather 

 mention them than refer to the ancient investiga- 

 tion upon Pulmonatse Prof. Vogt has traced these 

 investigations in a species of Action more exten- 

 sively than anybody else. He has noticed that the 



[PLATE LIL ACTJEON.J 



division (Plate LII,fig. E,) of the yolk goes to form 

 a germ consisting of homogeneous cells and that 

 after many more than twenty-four cerls had been 

 formed the external or peripheric cells assume a 

 somewhat different aspect from the internal 

 which would centre in the interior. And at that 

 time the peripheric cells (Fig. F) would forma 

 sort of envelope to the inner cells and then a di- 

 vision take place in the inner mass so that here al- 

 so the body assumes very soon a bilateral syme- 

 trical disposition. But what is curious is that on 

 the sides of the anterior portion of the body, (Plate 

 LII, fig. I) there are remarkable rotary appenda- 

 ges formed and between them a rudimentary foot. 

 The upper portion (Fig. G) of the body is soon 

 separated from the lower portion so that before 

 the animal leaves his shell, we have (Fig. H) an 

 upper part and a lower part and lateral wheels, by 

 which the animal moves like the Rotifera, and a 

 sort of foot and a sac (Fig. J) containing the va- 

 rious organs. Then the shell begins to be devel- 



