INTEGUMENT OF TUNICATA. 393 



ment of the richer gemmation, aud we must regard the sexual appa- 

 ratus as having been atrophied. This atrophy of the sexual apparatus 

 is referable to the increased reproductive activity of the stolon. 

 In the Salpas the progeny of the sexless generation are inva- 

 riably sexually complete, and so we have an example of pure 

 " alternatio generationis/' whilst in Doliolum the asexual reproduc- 

 tion is only played out after a numerous series of stolon-bearing 

 generations have succeeded one another. In this manner the condi- 

 tion of the Cyclomyaria approaches more closely to that of the 

 original Ascidiau-gemmation, and this both in the external position 

 of the stolon and in the mode of connection of the buds with the 

 stolon. The internal stolon of the Salpas, on the contrary, is widely 

 separated from the primitive arrangement, not only by its position, 

 but also by the assumption of the material of the stolon by the buds. 



Integument. 



§ 302. 



The investment of the body of the Tunicata consists in its 

 primitive condition of a cell-layer formed from the ectoderm. It 

 persists in this condition in the Copelata, in which it appears to 

 represent, at any rate to a large extent, the body-wall. The flattened 

 cells here form but a single layer. This simple aiTangement gives 

 place, in the higher division, to a complication ; the primitive 

 arrangement only occurring in a transient form in the earlier stages 

 of development. 



A layer of material secreted from the cells of the ectoderm forms 

 a covering enclosing the body, and known as the "mantle." This 

 condition is not altogether a new one, for in many Copelata a fore- 

 runner of the mantle is to be found. In them the cells in the 

 neighbourhood of the incurrent orifice attain considerable dimensions, 

 and secrete a slimy but consistent substance, which forms in large 

 quantities, and gradually enclosing the body more or less completely, 

 assumes the appearance of a basin-like structure of relatively large 

 dimensions. It was described by earlier naturalists as the ' ' house," 

 and functions as a protective organ for the body (Oikopleura). 



The secretory activity, which is here confined to a particular 

 region of the body-surface, is spread in the other Tunicata over the 

 whole superficies. Its products constitute the outer mantle, 

 which in its essential character belongs to the category of cuti- 

 cular structures. In consequence of the migration into it of form- 

 elements from the ectoderm, the resulting tissue assumes the 

 characters of the connective substances. The primarily homo- 

 geneous layer becomes converted into an intercellular substance. 

 The cells scattered in it present a great variety of characters. Often 

 the mantle thus formed attains a preponderancy over all other 

 organs, and serves, in consequence of its rigidity, as a supporting 



