MUSEUM OF COMPARATIVE ZOOLOGY. 343 



The life-history of a typical tunicate seems to be about as follows : 

 The tailed larva becomes converted into an animal, which may be- 

 come sexually mature, as in Ascidia, or may remain rudimentary, a3 

 in Pyrosoma, but which, in all cases, develop zooids by budding, and, 

 in most cases at least, discharges eggs into the bodies of these zooids, 

 which are hermaphrodite, and discharge their eggs into other buds in 

 the same way. The process of vegetative reproduction by budding 

 is well known to be antagonistic to true sexual reproduction, and 

 although it is very common among the lower representatives of most 

 of the larger groups of animals, it is not usually found to occur 

 among the higher forms, and is replaced by sexual reproduction, 

 unless there is some special necessity for its retention, as there is 

 in those forms which are fixed, as the Cirrhipeds and Ascidians. 

 Wherever these fixed animals are united into a colony, the power to 

 multiply by budding is of course essential, and however disadvanta- 

 geous it may be in other respects, it must be preserved, and we see that 

 in the Tunicata it has been so modified that instead of being antago- 

 nistic to, it has become to a certain degree accessory to reproduction 

 by eggs. Wherever a colony is arranged in a definite form, as in Py- 

 rosoma, there must be a point at which any further increase in size 

 would be of no advantage ; budding will therefore continue until this 

 point is reached, but we should expect that the last series of zooids 

 in each colony would gradually lose the tendencyto multiply in this 

 way. After this change had taken place, no more eggs could be dis- 

 charged from the body, and the ovary, being now of no functional im- 

 portance, would also tend to disappear. We should therefore have, 

 first, the tailed larval stage, then a variable but limited number of 

 hermaphrodite zooids, and finally, a zooid with the male organs only 

 developed, but containing an egg derived from the ovary of the one 

 next before it in the series. 



Suppose, now, that this series of zooids should become adapted to a 

 free locomotive life. The larval stage would disappear, as already 

 shown, and we should now have a series of hermaphrodite zooids, end- 

 ing with an egg-bearing male. The necessity for budding would 

 cease to exist as soon as the solitary locomotive life was entered upon, 

 and as this process is known to be antagonistic to high evolution and 

 great specialization, it would gradually disappear until the series had 

 been reduced to two, an hermaphrodite zooid hatched from the egg, 



