314 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



that the cause of the flexibility is a local thinning of the test 

 and a more fibrous structure of its matrix (k, ti ' , fig. 4). As 

 the change in thickness is always abrupt a very well defined 

 hinge is thus formed. In the contracted condition shown 

 in the figure the hinge appears at the bottom of a groove 

 which is of a narrow V-shape in the younger individuals. 

 In the older ascidians the width increases greatly, so that 

 the cross-section, in the contracted condition, shows the 

 hinge bent nearly double and projecting below the ventral 

 surface of the test. An early stage in this widening of the 

 groove and hinge is shown at h, fig. 4. In this condition 

 the approximation of the edges of the groove and the 

 foreign matter that accumulates in it make the boundaries 

 of the plates quite conspicuous. When on the other hand 

 the muscles of the animal relax, the elastic hinges straighten 

 out, the grooves in which they were located become broader 

 and shallower, and the outlines of the plates are conse- 

 quently much less distinct. The line that bounds the plates 

 peripherally (J>, fig. 3) has not the same character as the 

 hinges, but merely represents an irregular local thinning 

 and wrinkling of the test. 



The Test Matrix. 



The consistency of the test is cartilaginous, but, in the 

 disk especially, it is somewhat brittle, cracking when bent 

 much. 1 The matrix is composed of two very distinct 

 layers; an inner, of animal cellulose or tunicin, and an 

 outer one which is not composed of this substance and 

 is much thinner (fig. 4). 



The tunicin layer is translucent or transparent. Its 

 matrix is for the most part homogeneous, but fibres are 

 quite plentiful in certain rather variable regions. They 

 always occur quite abundantly in thinnest parts of the test, 

 especially the hinges, and are sporadically encountered in 

 other places. In the disk their direction is parallel to the 

 surface, but in the other parts of the test quite a few 



1 It has been said so often that the test of C. macleayanum is horny that one would be 

 tempted to consider the two species quite different in this respect were it not for the 

 description of Swederus (1887) which states that the test is leathery. 



