FOEM OF THE BODY OF TUNICATA. 391 



reproduction whicTi leads to tlie production of Colonies or Stocks. 

 Gemmation is tlie fundamental process in this phEenomenou. It pre- 

 dominates in tlie Acopa. In many Ascidia a new individual buds 

 from tlie body of tlie adult animal, the parent sending forth a 

 runner (stolo) which is composed of form-elements belonging both 

 to ectoderm and endoderm. From this there is gradually differen- 

 tiated an organism identical with the parent animal. In this way 

 colonies of Ascidians (Ascidige sociales) are produced. In others this 

 proceeding appears with its successive steps compressed at a very 

 early period of life, and accordingly there buds forth from the 

 embryonic body of one Ascidian a second (Didemnum). Thus we 

 get two persons closely united to one another. From this case, it is 

 easy to derive those conditions in which the sessile young individual 

 gives rise by budding to a number of persons (Botryllus). In these 

 several generations, all produced by budding, follow one another. 

 From the first a bud grows out, which, like Didemnum, gives rise by 

 budding to two persons, and from these in like manner four arise. 

 When the mother-animal dies down, the eight budHngs are left in 

 communication with one another by means of the cloaca, and form a 

 rosette-shaped group. Such and similar processes give rise to the 

 colonies which we find among the Ascidite composite. The com- 

 bination of the scattered groups takes place through the agency of 

 a tissue belonging to the integument which in those persons which 

 live isolated lives is known as the mantle (outer mantle). 



By means of a peculiar method of grouiDing, the persons of the 

 Lucia3 form cylindrical colonies. The wall of the hollow cylinder is 

 formed by the ascidian-like persons and their common investment. 

 On the outer surface of the cylinder are found the entrance- 

 orifices ; opposite to them, and opening into the internal cavity, are 

 the excurrent orifices. The multiplication of the persons of the 

 colony is carried on by budding. The formation of new colonies is 

 provided for by the sexual reproduction. From the egg arises an 

 embryo, from which at once four persons bud forth. They remain 

 invested by the mantle of the first, and constitute as soon as they are 

 born a new colony. 



Since the persons which develop from an egg never in the com- 

 pound Ascidians acquire sexual organs, and since these organs on 

 the contrary appear in those persons which arise by gemmation, we 

 have here an example of the phenomenon known as Alternation of 

 Generations. 



What is performed in the Ascidise by means of offshoots starting 

 from the surface of the body, is carried out in the Cyclomyaria and 

 Thaliadge by a special organ — the germ-stock or stolo prolifer. It 

 exists also in the Lucise, but has there a more limited functional 

 activity. In the Cyclomyaria it appears as a process springing 

 usually from the dorsal surface of the body in the neighbourhood of 

 the excurrent orifice ; in the Salpse and in the Pyrosoma3 it has a 

 ventral origin, and agrees with the condition found in Cyclomyai'ia 

 only at the first, for, instead of budding out externally, it takes up a 



