436 ZOOLOG Y 



the posterior end it is produced into various processes and 

 lobes which fit into crevices of the rock, etc., 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. 



