LKCTURES ON EMBRYOLOGY. 



91 



[PLATE L SALP.E OR SOFT-SHELLED MOLLUSCA 

 ACEPHALA.] 



not all compound animals. Salpaa are soft shelled 

 Mollusca, in which the transverse muscular fibres 

 (Fig. C) are very distinct, and of which two kinds 

 are observed. And this is the peculiarity of Sal- 

 PSB, that some of them are constantly found to 

 form long chains of distinct individuals, united by 

 peculiar appendages,' and in two rows united side 

 by side, and back by back, so that in a chain there 

 are always two rows one (Plate L, fig D) in which 

 individuals are placed side by side ; another where 

 two such rows are united by their backs. These 

 compound individuals swim freely about in connec- 

 tion together, and are never known to separate or 

 live isolated, except, perhaps, after accidental sepa- 

 ration. But there are other Ascidians observed 

 which move free, and which are never found to 

 unite, but which, nevertheless, in other respects, 

 resemble so closely the former, that in tracing the 

 internal anatomy, no difference whatever is ob- 

 served. The arrangement of muscles, for instance, 

 in such a compound Ascidia (Plate L, fig. A), or 

 the arrangement of muscles in such isolated indi- 

 viduals, (Plate L, fig. C,) is identical. The size of 

 the individuals is even so similar, that this resem- 

 blance has struck observers ever since the Salpse 

 have been studied. 



Chamisso, the poet naturalist, who accompanied 

 Admiral Kotzebue around the world, ascertained 

 that there was among these animals a most extra- 

 ordinary mode of reproduction ; that this resem- 

 blance of individuals, attached and free, could be 

 fully accounted for. He found that within those 

 compound Ascidian Salpag, there were only isola- 



ted eggs developed, as you can see, (Plate L. fig. 

 A) ; that in the internal cavity there is one single 

 egg developed from the main cavity in each of the 

 compound individuals. And Chamisso has seen 

 those eggs born, developed and transformed into 

 isolated Salpae, which would grow to the size of 

 their parents, and when fully developed would not 

 produce isolated eggs and isolated individuals, but 

 a chain of individuals (Plate L, fig. C) arranged in 

 a similar manner to the compound animals, and 

 growing till they are born as a chain, and finally 

 developing to the size of their grandparent, with- 

 out separating, and living (as long as the observa- 

 tions were traced) in this compound arrangement, 

 to reproduce in themselves isolated eggs, without 

 ever one generation resembling the preceding. So 

 that the compound Ascidians would always pro- 

 duce isolated eggs, from which free individuals are 

 born 5 and those free individuals would always 

 produce chains containing numerous individuals, 

 which individuals would never separate in life, but 

 each of which would reproduce free ones. 



Over forty years these facts have been known 

 and fully described. Chamisso has traced these in 

 more than one instance without one link in the 

 investigation escaping his attention. Nevertheless 

 these facts were so astonishing, so different from 

 every thing that was known in the other classes of 

 the animal kingdom, that, up to this present mo- 

 ment, they are not generally believed or under- 

 stood. There are recent publications dated from 

 last year, in which these statements are not admit- 

 ted ; though the accuracy of Chamisso is unques- 

 tioned among Naturalists he having published 

 other investigations which show how accurate an 

 observer he is; and even after the investigations 

 of Chamisso have been confirmed by other obser- 

 vers, there is still doubt entertained upon the cor- 

 rectness of the views derived from those facts. 



Dr. Krohn, a German Naturalist, has traced the 

 same phenomena in some species, which he obser- 

 ved on the shores of Italy. He has traced, as 

 Chamisso did, their whole series of alternate gene- 

 rations without one single interruption. Steenstrup 

 has traced similar changes. These facts have even 

 been the starting point of his views upon alternate 

 generations, of which I have spoken more at length 

 before. Still more recently Mr. Sars, of whom I 

 have so often spoken, bas published the complete 

 history of the alternate generations of several spe- 

 cies of Salpa, in which the whole development 

 through alternate generations is studied and con- 

 firmed, so that we have no longer any ground to 

 doubt these observations. We must come up to 

 the conclusion that there are alternate distinct 

 generations in various classes of animals. We must 

 admit that there are animals in the Mollusca as 

 well as in the other departments, in which the 

 young never resemble the parent, but resemble 

 constantly and throughout life their grandparent, 

 as alternately these generations of compound and 

 free Salpa are observed. 



