TEGUMENTARY ORGANS 409 



characteristic portion of the Ascidian test ; it is next necessary to 

 notice the inner and outer surfaces ; the former of which is ordinarily 

 said to be covered by a cellular epithelium, the latter by a more or 

 less structureless horny epidermic layer. 



The so-called epithelium is, I believe, in all cases merely the 

 innermost unmetamorphosed layer of the ecderon, corresponding with 

 the rate Malpighii of the " epidermis " of higher animals. As the 

 Ascidian integument is ordinarily examined (i.e. in spirit specimens), 

 it is in the condition of the macerated integument of one of the higher 

 animals, and just as the " epidermis " of the latter may or may not, if 

 stripped off; bring away with it the deepest layers of the rete, so the 

 Ascidian test, when detached from the outer tissue, may or may not 

 retain the corresponding structure. 



The horny so-called " epidermis," on the other hand, is a structure 

 well worthy of attention, as a similar element is, as we have already 

 seen, to be met with in the widely different integuments of other 

 Mollusca. In all Ascidians I have found the outermost surface to be 

 formed by a structureless homogeneous layer, which contains less 

 cellulose than the subjacent tissue, and often has a brownish horny 

 aspect. In many Salpje, Phallusi^e, and Cynthiae, this outer layer 

 constitutes merely a tough wrinkled investment. In others (Synoicum, 

 Boltenia) it is prolonged with the subjacent layer into spines and 

 processes, but without being much thickened. In other Boltenise 

 again, and in various Cynthiae, it is greatly thickened, and almost by 

 itself constitutes large spines or even tesselated plates. In Cynthia 

 papillata {fig. 314. B), the whole outer surface of the test is covered 

 with spines {a), whose bases expand into polygonal plates, which 

 strongly resemble the spines of the Rajidae, to which reference will 

 be made below. The brown substance here appears to have invaded 

 the subjacent tissue, leaving spaces for the pre-existing endoplasts, so 

 as to give rise to a structure precisely resembling the bone of the 

 Plagiostomes, while to complete the resemblance, the pointed 

 extremity of the spine is marked by lines which pass from its 

 central cavity, parallel with one another, to the surface. I am not 

 sure that there are tubes, but otherwise the appearance is exactly that 

 presented by the pseudo-dentine of the integumentary spines of the 

 skate. 



It would appear, according to Milne Edwards, and the late 

 observations of Krohn, that the rudiment of the mantle exists in the 

 ovum of Phallusia before the cleavage of the yolk commences, as a 

 structureless pellucid coat, containing solitary or aggregated greenish 

 cells ; and it would seem as if the outer structureless layer with which 



