250 M. Van Beneden on the Physiology of the Simple Ascidians. 



l)ut rather by an examination — mature, comparative, and based 

 on the genesis of organization. It is four years since I studied 

 with care the development of this product in the Alcyonella — nor 

 has the delay in the publication of that work depended on me — 

 and since then the facts in general have come to the support of 

 my views. 



It has been said that reproduction by buds in some of the As- 

 cidi(B is a very recent discovery ; and Milne Edwards assumes, for 

 the basis of his classification, the twofold mode of reproduction 

 by eggs and by buds. But in 1761 Bohadsch had observed the 

 gemmiferous reproduction, and that even in a simple Ascidian, 

 the A. intestinalis. Hence it was that, in the ^ Encyclopedic Me- 

 thodique,^ Bruguiere wrote, — ^' It is probable that, independently 

 of their multipHcation by eggs, they enjoy also a propagation 

 by the integuments, according to the observations of Bohadsch 

 and Miiller.'^ 



The able Norwegian naturalist, Sars, has made some very cu- 

 rious observations upon the mode of formation of the compound 

 or aggregated Ascidia, which perfectly explain the symmetrical 

 arrangement of these little animals. Milne Edwards does not 

 admit the plurality of germs to explain the aggregation, and he 

 believes the formation by buds is sufficient to do so. Becently 

 several important facts have appeared in support of the assertions 

 of Sars, and it seems to me that doubt can no longer rest on the 

 exactness of his observations. Some animals in their embryo con- 

 dition, and before the adult form is reached, can divide themselves 

 and disaggregate, by a natural fissiparous reproduction, into se- 

 veral other individuals, which sometimes remain grouped together 

 and constitute a ready-made colony {Ascidice composite), and 

 sometimes they separate to live freely (Campanularia, Medusae j 

 &c.). An animal of the lower classes can thus reproduce itself 

 in its young age when it has still the embryo form : it dies in 

 giving birth to another generation before having attained adult- 

 ness, and that second generation has not passed through the 

 same phases of the mother that gave birth to them. 



To explain the passage of the water from the respiratory ca- 

 vity to the anal tube, openings or stigmata between the branchial 

 vessels have been supposed necessary, but I rather coincide in 

 the opinion of the naturalist who has lately denied the existence 

 of these communications. I have always seen a thin membrane 

 between the vessels ; and the communication, in my opinion, is 

 effected by an interruption of continuity between the parietes 

 which separate the respiratory cavity from the cloacum. 



Without having recourse to an alternating generation, as a 

 learned Dane, Steenstrup, has lately advocated in a small but 

 very remarkable book, it is easy, if we do not deceive ourselves. 



