ALTERNATION OF GENERATIONS IN THE TUNICATES. 513 



founding new colonies. Such a condition is found, for example, in 

 the Ascidiozooids ofPyrosoma, which produce stolons and also maturate 

 genital products. 



The distribution of these two kinds of reproduction among various 

 individuals of the colony must be regarded as a later, derived condi- 

 tion through which the first individuals arising in a colon}' then 

 became adapted exclusively for increasing the colony by budding, 

 while the lateral individuals formed new colonies by sexual reproduc- 

 tion. Such an arrangement, in which we see the first commencement 

 of alternation of generations, is found in the composite Aseidians. 

 Ganin, following the investigations of Krohn, established that, in 

 this case, the individuals developing from the egg are capable of 

 asexual reproduction only, and in this way lay the foundation of 

 the colonies, while the descendants of these individuals, produced 

 by budding, again develop genital products. 



In Sitl [in we find this condition more marked, and established 

 as a regular alternation of two generations, one reproducing only 

 asexually and the other only sexually. At the same time the two 

 generations vary to a certain degree in the structure of the body. 

 Grobben has rightly pointed out that these variations may be 

 explained by the different conditions of life and the different work 

 to be carried out by the two generations. The points that project 

 at the posterior end of the " nurse" of Salpa democratica-mucronata 

 (Fig. 262) serve as a protection for the proliferating stolon which 

 occurs in that region of the body. This "nurse*' of S. democratica- 

 mucronata further possesses one muscle-hoop more than the sexual 

 animal, and this is explained on the ground that, owing to the 

 presence of the massive proliferating stolon, greater muscular power 

 is required in this case to enable the animal to swim with equal 

 rapidity. On the other hand, the form of the sexual indi vidua 

 (that belonging to a chain) may be explained by the crowding of the 

 buds on the chain. 



It follows from the above that the solitary form and all the indi- 

 viduals of the chain produced by it must be regarded as members of 

 one and the same colony. The solitary (" nurse ") form is the 

 founder of the colony, while the individuals of the chain produced 

 asexually give rise to the foundation of new colonies in producing 

 sexually new solitary forms. 



The heteromorphous development of the individuals of the Salpa 

 colony recalls the polymorphism of the Siphonophora. Polymorphism 

 is still more strongly marked in the Doliolidae, where we have, in 



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