436 ZOOLOGY 
the posterior end it is produced into various processes and 
lobes which fit into crevices of the rock, ete., and serve to fix 
the animal; the true skin or dermis is continued into these 
processes. The gelatinous substance which composes the test 
of Ciona contains numerous cells split off from the ectoderm ; 
in the genus in question many of these cells soon die, but in 
others they live for some time, forming fusiform or stellate 
cells, often pigmented, or they develope a large vacuole (Fig. 
252), or in some genera they secrete calcareous spicules. The 
test is undoubtedly a cuticle secreted by the ectoderm, but it 
is kept alive to a certain extent by the cells which wander 
into it, and by the blood-vessels which make short incursions 
into it. The test contains in many instances a chemical 
substance identical or closely allied to cellulose, a substance 
rarely met with in animals, but almost universal in plants. 
Kowalevsky has recently shown that in some species the cells 
which wander into the test arise from the mesoderm, and pass 
through the ectoderm on the way to their final resting-place 
in the test. 
The ectoderm, which secretes the test, forms a single layer 
of cubical cells; beneath this is a layer of muscles arranged 
in large longitudinal bands, with few anastomoses, and 
numerous small transverse bands, which anastomose freely. 
Nerves and blood sinuses also ramify in the skin. 
The general disposition of the organs of the body in a 
simple Ascidian is as follows: the mouth leads into a large 
branchial sac, which extends throughout four-fifths of the 
body; this is pierced by very numerous slits, which, instead 
of opening directly to the exterior, communicate with a large 
chamber, the atrial cavity, which completely surrounds the 
branchial sac, except along the middle ventral line, and which 
opens to the outside world through the atrial pore. The 
atrial cavity is produced originally by an invagination from 
the exterior, and is consequently lined throughout by ectoderm. 
Behind the branchial sac, occupying the posterior fifth of the 
animal, is a closed space, in which the stomach, heart, and 
generative organs are enclosed; the intestine and ducts of the 
reproductive glands run up the dorsal surface of the branchial 
sac, and open into the atrial chamber, the latter near its pore. 
