48 ALTERNATE GENERATION 



which the SalpöB belong, is their mode of propagation as 

 confirmed by the observations of Milne Edwards. Al- 

 though all the Ascidice are affixed, yet their young swim 

 about freely, and to this end their oval obtuse bodies are 

 provided with a special organ for swimming, or sort of 

 tail which they move about in the same way as tadpoles 

 and Cercaria. At all events, this appendage has been 

 observed in all the Ascidice which are aggregated in 

 smaller swarms, or which form larger colonies, in which 

 latter, the individuals are so closely connected together by 

 means of a common, gelatinous, or membranous substance 

 which surrounds them (enveloppe,) that one is inclined to 

 consider them as immersed in it. In several allied genera, 

 moreover, we find more or fewer individuals of the same 

 form and size arranged in a very regular manner, 6, 8, 1 0, 

 such being grouped on each side of a straight line or 

 radiating around a common central point, (as for instance, 

 Bofryllus,) and forming a hollow or pit common to all, 

 and to the construction of the sides of which all the 

 Ascidice fixed around the common centre contribute, and 

 in which are situated the anal and generative openings, so 

 that in consequence of this structure, the mature ova and 

 young ones must be collected in the hollow or pit before 

 they escape into the water. It is very probable, that the 

 young remain some time in this receptacle, in order as it 

 were to be hatched, before they swim about ; at least, 

 great numbers of them are frequently retained in it. 



That the progeny when it quits this receptacle, does so 

 in the form of caudate free swimming animalculse, was 

 observed as far back as 1828, by Audouin and Milne 

 Edwards, and independently of them, by Sars (Beskii- 

 velser og Jagttagelser, 1835,) and the observation is 

 confirmed by Dalyell, (The New Edinburgh Philos. 

 Journal, January 1839.) But for our first clear idea of 

 the complete development of these free swimming ani- 

 malculse in their earliest period, we are indebted to the 

 above-mentioned larger memoir by Milne Edwards, and 

 the observations made by him in 1834 and 1839. After 



