30 METAGENESIS. 



of generations, in that the sexually produced larva is asexual, and, 

 after a series of asexual generations, produced gemmiparously, there 

 appear sexual generations, which however continue to reproduce 

 themselves by budding. 



The type of alternations of generations observable in Botryllus 

 becomes, as pointed out by Huxley, still more marked in Pyrosoma. 



The true product of the ovum is here (vide p. 21) a rudimentary 

 individual called by Huxley the Cyathozooid. This gives rise, while 

 still an embryo, by a process equivalent to budding to four fully 

 developed zooids (Ascidiozooids) similar to the parent form, and itself 

 dies away. The four Ascidiozooids form a fresh colony, and repro- 

 duce (1) sexually, whereby fresh colonies are formed, and (2) by 

 ordinary budding, whereby the size of the colony is increased. All 

 the individuals of the colony are sexual. 



The alternation of generations in Pyrosoma widely differs from 

 that in Botryllus in the fact of the Cyathozooid differing so markedly 

 in its anatomical characters from the ordinary zooids. 



In Salpa the process is slightly different 1 . The sexual forms 

 are now incapable of budding, and, although at first a series of 

 sexual individuals are united together in the form of a chain, so 

 as to form a colony like Pyrosoma or Botryllus, yet they are so 

 loosely connected that they separate in the adult state. As in 

 Botryllus, the ova are ripe before the spermatozoa. Each sexual 

 individual gives rise to a single offspring, which, while still in the 

 embryonic condition, buds out a ' stolon' from its right ventral side. 

 This stolon is divided into a series of lateral buds after the solitary 

 sexual Salp has begun to lead an independent existence. The solitary 

 sexual Salp clearly corresponds with the Cyathozooid of Pyrosoma, 

 thouo-h it has not, like the Cyathozooid, undergone a retrogressive 

 metamorphosis. 



By far the most complicated form of alternation of generations 

 known amongst the Ascidians is that in Doliolum. The discovery of 

 this metamorphosis was made by Gegenbaur (No. 10). The sexual 

 form of Doliolum is somewhat cask-shaped, with ring-like muscular 

 bands, and the oral and atrial apertures placed at opposite ends of the 

 cask. The number of gill slits varies according to the species. The ovum 

 oives rise, as already described, to a tailed embryo which subsequently 

 develops into a cask-shaped asexual form. On attaining its full size 

 it loses its branchial sack and alimentary tract. While still in the 

 embryonic condition, a stolon grows out from its dorsal side in the 

 seventh intermuscular space. The stolon, like that in Salpa, contains 

 a prolongation of the branchial sack 2 . 



On this stolon there develop two entirely different types of buds, 

 (1) lateral buds, (2) dorsal median buds. 



1 Vide p. 28. 



2 I draw this conclusion from Gegenbaur's fig. (No. 10), PI. xvi., fig. 15. The 

 body (.r) in the figure appears to me without doubt the rudiment of the stolon, and 

 not, as believed by Gegenbaur, the larval tail. 



