W. K. BROOKS ON THE GENUS SALPA. 233 



Seeliger's conception of the nature of the process of budding forces 

 him to regard these latter cells as " mesoderm," and I shall not discuss 

 this subject, but shall confine myself to a request that the reader study 

 his figures and form his opinion of the nature of these cells from them, 

 although Seeliger himself says (p. 639) of the one I have copied, " In dem 

 distalen Teil des Geschlechtsstranges findet man oft schon fruhzeitig 

 neben den indifferenten Zellen solche, welche durch den grossen, 

 blaschenformigen Kern sich als Eizellen zu erkennen geben (Fig. 22) und, 

 wie die weitere Entwicklung lehrt, in der That zum Teil zu Eiern 

 werden," and I think the question of the migration of the eggs of 

 pyrosoma may be dismissed with this quotation. 



I believe that the facts admit of only one interpretation, and prove 

 that both salpa and pyrosoma are descended from an hermaphrodite 

 ancestor which was very similar to the modern ascidians in structure 

 and habits, p. 123, although it was, no doubt, more primitive than any 

 modern ascidian, p. 128. When mature, the animal which hatched from 

 the egg was hermaphrodite, and it gave rise, by budding, to hermaphro- 

 dite descendants which also produced hermaphrodite buds, and so on. 



Whether the method of budding which is exhibited by doliolum, 

 pyrosoma and salpa was the primitive method for all the tunicates or 

 not, there can be no doubt that the common ancestor of these three 

 groups gave rise on the middle line of the ventral surface to buds which 

 were symmetrical in the middle plane of the parent, and that the diges- 

 tive organs and perithoracic organs of all the buds were derived from 

 the corresponding parts of the parent, and were therefore, in ultimate 

 origin, part of the corresponding organs of the animal which hatched 

 from the egg.* The hermaphrodite reproductive organs of the buds 

 were derived in the same way from that of the first member of the 

 series, and were, in their ultimate origin, part of it. 



Before pyrosoma and salpa diverged from the ancestral form, each 

 member of the series acquired the habit, so characteristic of pyrosoma, 

 of passing eggs into the body of a bud to ripen and develop there. 



At first all the individuals which make up the life-cycle were alike, 

 both physiologically and anatomically, and there was no polymorphism. 



In pyrosoma this indefinite series of similar zooids has been differ- 

 entiated into a first zooid, which has become rudimentary, and is now 

 known as the cyathozooid; a second generation of four infertile or 



* In a future paper I shall discuss the relation between the budding of Salpa, 

 Pyrosoma, and Doliolum, and that of Botryllus, Botrylloides, and other ascidians. 



