480 Miscellaneous^ 



I reserve for a more detailed memoir the investigation of the for- 

 mation, segmentation, &lc. of the single ovum, and I shall confine 

 m5-self at present to calliiigattentionto some points of the organization 

 of the hatched tadpole-like larva. The large vesicle vphich I regarded 

 as the first rudiment of the common cloaca has certainly this phj-sio- 

 logical signification ; but its morphological importance is greater than 

 I had supposed. 



This part, in fact, possesses the value of an individual ; that is to 

 say, it is the homologue of the Cyatliozoid of the embryo of Pi/ro- 

 soma. The arrangmenfc of the other Ascidians relatively to this 

 vesicle is exactly the same as that of the young Ascidiozoids of 

 Pyrosoma relatively to the Cyathozoid. To be convinced of this 

 it is only necessary to compare Kowalevsky's fig. 54, pi. xli. with 

 the figure given by me for Astellium, in my ' Recherches sur Ics 

 Synascidies ' (pi. xxvi. fig. G). To render this comparison perfect 

 we must reverse the figure of the Pyrosomn, and turn it 45° from 

 right to left round a longitudinal axis. The presence of a very 

 abundant white pigment renders the continuous observation of the 

 embryos of the Diplosomidae very difficult, and prevented my per- 

 ceiving this remarkable agreement. 



The differences of structure which exist in the adult state between 

 the branchife of Astdlinm and Pyrosoma are in relation to the 

 different modes of existence of these animals. Moreover the em- 

 bryos of an allied group, the Botryllidaj, have a branchia which 

 astonishingly resembles that of Pyrosoma. 



We may therefore regard the DiplosomidjB as reiiresenting the 

 fixed state of a type of which Pyrosoma is the swimming or pelagic 

 form. Consequently the group Luciee of Savigny may be divided 

 into two families, Pyrosomidne and Diplosomida>, presenting recipro- 

 cally the same relations as the 8iphonophora and the Hydriformes 

 among the Aealephan Ca'lentcrata. 



A last fact which is important to indicate is, that in the pecu- 

 liarities of the development of the Luciae (defined as we have just 

 seen) we find a new application of the law enunciated by ns as 

 the consequence of our embryogenic investigations on the group 

 Molgulidse. The Pyrosomidte, which live free, present an abridged 

 and condensed development, a partial segmentation, and an anurous 

 embryo destitute of organs of sense ; while the sedentary Diploso- 

 midse in the adult state have a dilated metamorphosis and a 

 urodelous embrj-o, furnished with a well-developed visual and audi- 

 tory apparatus. I may add that the tadpole-like larva of Astellium 

 sponyvforme possesses a caudal appendage, the musculature of which 

 is very complex, while its membranous pai't is traversed by horny 

 filaments, like those described by us in the simple Ascidians of the 

 group Cynthia and in the Hynascidians of the genera Botryllns and 

 Botrylloides. 



Lastly, in Astellium, as in Ascidia scahra, Miiller, and A. gelatinosa, 

 Pdsso, the tunic of cellulose is formed independently of the embryo, 

 during (and even before) the segmentation of the vitelkis. However, 

 this process is less distinct, than in the Ascidians in which we have 

 observed it. — Comptes licndus, December 13. 1S75, pp. 1214-1216. 



