512 TUNICATA. 



;i common efferent duct that opens out into the atrial cavity between 

 the intestine and the stomach on a papilla-like prominence (see 

 Salensky, Nos. 101 and 102, and Seeligek, No. 105). The testis 

 develops comparatively late in the chain-form. 



5. The Interpretation of the Alternation of Generations 

 in the Tunicates. 



Alternation of generations is found among the Tunicates in a 

 marked form in Pyrosoma, Doliolnm and Salpa. This fact has 

 long attracted the attention of zoologists, who have attempted to 

 explain it in many different ways. We shall adopt the view first put 

 forward by Leuckart (No. 98) and accepted later by Claus ;: and 

 Grobben (No. 79) that alternation of generations in the Tunicates 

 must lie regarded as having arisen in consequence of the formation of 

 stocks through division of labour, and we shall follow Grobben's 

 clear exposition of this view. Among more recent descriptions we 

 may specially mention those of Uljanin (No. 86) and Seelicjek 

 (No. 106). 



The Larvaeea, which are conjectured to be the most primitive of 

 all existing Tunicates, develop through sexual reproduction. Tins 

 seems to suggest that the capacity for asexual reproduction (by means 

 of buds) was acquired as a consequence of the attached manner of 

 life. Asexual reproduction is indeed very common among attached 

 animals. We may, with Grobben, suggest as the cause that the 

 abandonment of locomotion left a larger proportion of the substance 

 of the body available for reproduction, so that it was possible for 

 attached animals to introduce a new method of multiplication into 

 the cycle of their development. It may also, however, lie added that 

 when, in consequence of attachment, cross-fertilisation became more 

 difficult, the capacity for asexual reproduction would become of special 

 importance for the maintenance of the species. 



It is evident that, originally, all the individuals of forms which had 

 developed this character were equally able to reproduce themselves 

 either sexually or ase.xuallv. 



Asexual multiplication led to the formation of stocks, the buds 

 being either altogether incapable of separating from the parent or 

 else able to do so only incompletely. All individuals were thus at 

 first capable, by asexual multiplication, of increasing the size of the 

 colony to which they belonged, or else by sexual multiplication of 



* Grundz. d. Zool. 4 Aufl. 



