68 



PROF. AGASSIZ'S 



dergone all its changes, is the formation of a lay- 

 er of more animated substance, the beginning of 

 the young animal. We have here (Plate III, fig. 

 B) the germ as it flattens out at one end and is 

 contracted at the other part, divided as it were in- 

 to two connected discs, the larger assuming after- 

 ward another form (Fig. C), the smaller one grow- 

 ing laterally, when soon it is observed what has be- 

 come of these two extremities of the expanded 

 disc (Fig. D). One will be the head end of the 

 germ, and the other will be the caudal end of the 

 germ. Those serratures upon the posterior ex- 

 tremity of the animal, represent the divisions 

 in the animal layer, in the blastoderma, or germ, 

 which will give rise to the joints or rings of the 

 chest ; while the anterior disc will represent that 

 part of the body which properly forms the head, 

 growing larger and larger; these flat discs are drawn 

 backwards, forwards and on the side, so that it 

 gradually surrounds the yolk, having assumed a 

 more elongated shape (Fig. F) leaving the mass of 

 the yolk free at the dorsal side, so that when 

 seen from above (Fig. G), you have the margin of 

 the animal in sight, which is rolled over the yolk. 

 We have also here the eyes, which are forming at 

 the anterior portion of the germ ; and also the in- 

 dications of the formation of a heart 



But from below (Plate III, fig. E,) we see how 

 the lower surface is changed ; the formation of 

 those parts which will represent the mouth, is 

 seen, and also the formation of those parts which 

 will represent ihe legs, and in addition, the parts 

 which will represent the tail. And those separ- 

 ations of different joints become gradually more 

 and more distinct, (Fig. G,) so that upon close 

 examination, you may find that the germ is now 

 a little animal, which soon escapes under the form 

 of fig. H. Here we have the young, which rises 

 from such a transformation ; and this young is the 

 young of a Palasmonof the character of Plate 

 XVII, fig. C. The young as it is hatched represents 

 the figure which is a general characteristic, 

 not only of the Macrouran Crustacean, but it has 

 more particularly the form of those Entomostraca 

 which have been described under the name of 

 Cuma (Plate XIX, figs. D and E) I have traced 

 many of those which occur in Boston harbor, 

 of Palasmon, of Hippolyte, even of Mysis, 

 and they all give rise to young which are species 

 of the genua Cuma, belonging to the Entomo- 

 straca of Carcinologists ; showing that there are 

 still extensive grounds to cultivate in the history 

 of Crustacea, and that they undergo metamor- 

 phoses. The subject of the metamorphoses of 

 Crustacea has been discussed very extensively, 

 Rathke denied positively that there are metamor- 

 phoses among Crustacea; while facts were col- 

 lected in Ireland which showed distinctly that such 

 metamorphoses take place. 



Mr. J. V. Thompson, who has published many 

 interesting investigations upon the lower Marine 

 aaimals the same to whom I have before referred 



and who discovered that the young Comatula 

 had a stem in its earlier condition, was also the 

 first to notice that the so-called Zoea (Plate XIX 

 fig. A and B), were not animals of a peculiar ge- 

 nus, but that they were the young of Crabs of 

 Crabs of similar form to that figure, (Plate XVII, 

 fig. A.) Captain Tuckey of the British navy, ob- 

 served similar changes. He saw the transforma- 

 tion of the egg into those entomostracal germs,and 

 further changes, which left no doubt in his mind 

 that the Crabs underwent the above described met- 

 amorphoses. 



The objections of Rathke arose from the fact.that 

 the Crawfish, a Crustacean, in which he studied 

 that embryology, does not undergo extensive chan- 

 ges of form during its embryonic growth. The 

 young Crawfish resembles very early the perfect 

 animal; so that by correct investigations this emi- 

 nent Embryologist was misled ; though he after- 

 ward acknowledged his error with reference to the 

 investigatioas of Thompson, in the most liber- 

 al and generous manner. These metamorphoses 

 have been traced extensively in other Crustacea. 

 Zaddach has published a monograph, in which 

 he has represented the changes which this animal, 

 Apus, (Plate XX, figs. A to E) undergoes, from its 

 primitive formation in the egg, up to its perfect 

 condition, (Fig. E.) In the beginning (Fig. B) it 

 has but few appendages ; and afterwards, others 

 successively, more numerous, are added under- 

 neath. Here (Fig. F) is a diagram of another ani- 

 mal, the Achtheres, in which similar embryonic 

 changes have been observed. First, there are 

 also but few appendages, but afterwards several 

 pairs have been added to form the various appen- 

 dages which exist in the adult (Fig. G.) How 

 similar Rotifera are to these various embryonic 

 conditions of Entomostraca, will not escape the 

 observer, who is simply reminded of the existence 

 of these microscopic animals, (Plate XX, fig. 

 0.) They resemble most remarkably those 

 Entomostraca in their earliest condition. But in 

 their embryonic condition, Crustacea even 

 Crabs, as well as Lobsters have young which re- 

 semble perfect forms of those Entomostraca, be- 

 yond which certain Crustacea do not pass. We 

 have thus direct indication that they should be con- 

 sidered as the lowest ; and so would we place at 

 the lowest range, all the Rotifera and these vari- 

 ous kinds of Entomostraca and Parasites, (Plate 

 XX and Plate XVIII.) Next, we would have the 

 Malacostraca; and among them, those lowest, with 

 uniform rings, which are not combined into dis- 

 tinct regions; and next, those in which the rings are 

 also not combined, but the legs diversified, (Plate 

 XVIII, figs. A, B, C, E); and above all, those in 

 which the rings are combined in various ways, 

 which are still more diversified, (Plate XVII.); 

 placing the Lobster and Shrimp lower among 

 them ; but we should consider the Crabs (Fig. A) 

 the highest of all, because in these the concentra- 

 tion has gone to the extreme , the tail which was 



