INTRODUCTION. 7 



Lister made known, in 1834, his important observa- 

 tions on the circulation in a minute social Ascidian, 

 afterwards named by Wiegmann PeropJiora Listeri. His 

 remarks were published in the ' Philosophical Trans- 

 actions,' and prove that, as in Sertularia and Gam- 

 vannlaria (Hydroida), there is in this Ascidian a 

 circulation common to many individuals or to a whole 

 system of combined individuals. The author also gives, 

 in his admirable memoir, a more complete account of 

 the blood-circulation existing in the Tunicata than any 

 that had previously been published. The figures are 

 very good. The memoir is entitled " Some observations 

 on the Structure and Functions of Tubular and Cellular 

 Polypi and of AsciJice-" 



Then followed, in 1830 [in ' Weigm., Archiv.'], Sars' 

 very curious observations on the metamorphosis of 

 Botryllus, showing that one larva enclosed several 

 young, united and already arranged in order ; that it 

 is in fact not a mere individual but a compound system 

 of individuals swimming* freely about in the external 



i 



form of the usual tadpole larva. This extraordinary 

 fact has been verified by the subsequent observations 

 of many other naturalists. 



In 1842 Milne-Edwards' beautiful memoir entitled 

 " Observations sur les Ascidies Composees des cotes 

 de la Blanche ' was published. It was read before 

 the Academic des Sciences in 1839. In this memoir, 

 which will ever be looked upon as one of the most 

 admirable ever produced in this field of enquiry, the 

 anatomy and embryology of these animals are treated 

 at great length, and the author fully verifies the con- 

 clusions of Savigny. He supplies many points which 

 were left undetermined by that indefatigable observer 

 relative to the reproductive, vascular, and respiratory 

 organs. He also describes several new species, and 

 proposes to arrange the Ascidians in three divisions, 

 namely, the Simple, the Social, and the Compound. 

 The social Ascidians, however, pass by such a finely- 

 graduated series from the compound to the simple 



