34 APPENDICULARIA. 



Besides these parts there appears on the dorsal side a hollow tube, the 

 origin of which is unknown, which gives rise to the nervous system. 



The hollow process of the respiratory sack is purely provisional, and 

 disappears without giving rise to any permanent structure. The right and 

 left prolongations of the pericardial cavity become solid and eventually give 

 origin to the mesoblast. The ventral process of cells is the most important 

 structure in the stolon in that it gives rise both to the alimentary and respir- 

 atory sacks, and to the generative organs of the sexual Salps. The stolon 

 containing the organs just enumerated becomes divided by transverse 

 constrictions into a number of rings. These rings do not long remain 

 complete, but become interrupted dorsally and ventrally. The imperfect 

 rings so formed soon overlap, and each of them eventually gives rise to a 

 sexual Salp. Although the stolon arises while the asexual Salp is still in an 

 embryonic condition, it does not become fully developed till long after the 

 asexual Salp has attained maturity. 



Appendicularia. Our only knowledge of the development of 

 Appendicularia is derived from Fol's memoir on the group (No. 8). He 

 simply states that it develops, as far as he was able to follow, like other 

 Ascidians ; and that the extremely minute size of the egg prevented him 

 from pursuing the subject. He also states that the pair of pores leading 

 from the branchial cavity to the exterior is developed from epiblastic 

 involutions meeting outgrowths of the wall of the branchial sack. 



Metagenesis. 



One of the most remarkable phenomena in connection with 

 the life history of many Ascidians is the occurrence of an 

 alternation of sexual and gemmiparous generations. This alter- 

 nation appears to have originated from a complication of the 

 process of reproduction by budding, which is so common in this 

 group. The mode in which this very probably took place will 

 be best understood by tracing a series of transitional cases 

 between simple budding and complete alternations of gene- 

 rations. 



In the simpler cases, which occur in some Composita 

 Sedentaria, the process of budding commences with an out- 

 growth of the body wall into the common test, containing a 

 prolongation of part of the alimentary tract 1 . 



1 It is not within the scope of this work to enter into details with reference 

 to the process of budding. The reader is referred on this head more especially to the 

 papers of Huxley (No. 16) and Kowalevsky (No. 22) on Pyrosoma, of Salensky 

 (No. 35) on Salpa, and Kowalevsky (No. 21) on Ascidians generally. It is a question 

 of very great interest how budding first arose, and then became so prevalent in these 



