TUNICATA. 



1199 



and brown pigment-granules, are round, having 

 a diameter of O'OOS'" to O'Ol'", or elon- 

 gated, with a diameter of O'OOG'" to 0-008'" ; 

 others are of a pule colour, and, from the 

 double nuclei and the included cells (2 to 7) 

 seen in some of them, strongly resemble 

 the cartilage-cells of the superior animals. 

 This resemblance is rendered the more strik- 

 ing by the round or elongate form of these 

 cells, by the peculiar arrangement of the en- 

 closed cells, and by the union of some of 

 the mother-cells, in this case generally round 

 and smaller, into groups of two or four. 

 This resemblance, however, is only external, 

 and due only to the fact that the cells 

 increase by formation in the interior, as in 

 cartilage ; because more exact observations 

 show that these cells, by further transitions, 

 become identical with the simpler pigment- 

 cells described above, and are only the more 

 developed forms of the latter, the pigment 

 having disappeared by degrees on account of 

 the condition of their growth. 



The third layer is formed of a yellowish, 

 horny epidermis. The thin hairs which cover 

 the surface of the Cynthia papillata are formed 

 by this and the exterior fibrous layer. At 

 certain spots a bundle of fibres springs up from 

 the plane surface of the latter, which, being 

 coated with the horny epidermis, rises on the 

 surface as small needle-shaped bodies. 



Where the test of the Cynthia attains a thick- 

 ness of i to \\'" and more, its composition 

 often changes in a remarkable manner {fig. 

 776.). In this case the epithelium is succeeded 



Fig. 776. 



Transverse section of Cynthia paplll<ita, magnified 

 100 times. {After Lowly and Kolliker.) 



1, Internal layer of epithelium; 2, second layer, 

 homogeneous, with pigment-cells ; 3, third layer, 

 composed of alternate layers of radiating fibres and 

 fibres parallel to the surface of the test ; 4, fourth 

 layer, fibres parallel to the surface of the test ; 

 5, spines ; a, the thin yellowish outer surface ; b, 

 fibrous nuclei. 



by a clear, homogeneous, structureless mass, of 

 a moderate thickness, with scattered pigment- 

 cells and nuclei. Next is a fine fibrous tissue, 

 composed of a great number of thin layers of 

 circular fibres, without cells or nuclei, and of 

 radiating fibres that unite these layers ; this 

 pa>ses externally into an irregular fibrous 

 tissue, covered with a horny epidermis. Where 

 this peculiar stratification of fibres exists, the 

 test is not separable into lamina;, because the 

 radiating fibres firmly connect the thin layers 

 of circular fibres. 



Treated with hydrochloric acid and with 

 soda, the test of Cynthia is rendered quite 

 white. The pigment-cells, the coloured epi- 

 dermis, the mother-cells, the crystals, the 

 epithelium, the nuclei, and the granules are 

 dissolved, and there remain only the fibres 

 and the homogeneous substance that exists 

 here and there. These two elements, then, 

 are composed of cellulose. 



The structure of the thick test of the 

 Cynthia Canopus is very similar to the above. 

 Interiorly there is an epithelium, then a thick 

 layer of longitudinal and circular fibres, some- 

 what indistinctly stratified, in which, towards 

 the exterior, crystals and largish round bodies, 

 composed apparently of groups of cells, are 

 disseminated ; lastly, a thin layer of solid, 

 whitish epidermis, with little conical papillae, 

 usually accompanied by processes from the 

 fibrous layer. The thick fibrous layer, only, 

 resists the action of hydrochloric acid and of 

 soda ; all the rest is dissolved without any 

 residue. 



Cynthia pomaria presents, as the chief ma- 

 terial of its test, a layer of fibres similar to 

 those previously described, having chiefly a 

 longitudinal direction. Between the fibres 

 are crystals, round pigment-cells, measuring 

 O-OO^'to 0-006'", and further, here and there, 

 peculiar elongated cells, filled with yellow 

 granules, measuring O'OOS'" and more. In- 

 ternally, and adhering to the fibrous layer, is 

 a simple epithelium, with polygonal cells that 

 have diameters of Q'006"' to O'OOS'". This 

 adheres to the inner tunic by means of mus- 

 cular fibres. Externally the fibrous layer is 

 covered by a yellowish, solid layer of unde- 

 termined structure. In the interior parts of 

 the fibrous layer occurs a somewhat large 

 number of peculiar cells, apparently not ana- 

 logous to any other of animal or vegetable 

 structure. These cells are primitively similar 

 to pigment-cells, and round, but possessing a 

 thicker membrane, and without any apparent 

 nucleus. Subsequently they grow, preserving 

 their shape, to the size of O'Ol'". The mem- 

 brane at the same time continues to thicken, 

 so much so that the cavity of the cell increas- 

 ing but slightly, the membrane attains a thick- 

 ness of 0-004/". Lastly, the size of the cell 

 increases to 0'02'" and the thickness of the 

 membrane to 0'006'". Whilst this develop- 

 ment is going on, fine lines are observable in 

 the thickened cell-membrane, and ultimately 

 the membrane is transformed into fibres, so 

 that one may see the moderately sized cells 

 in their cavities, and yet occupied by pigment 



