REPORT ON THE TUNICATA. 



29 



by a delicate layer of protoplasm, and having a nucleus at one side, the whole of the 

 interior of the cell being a large vacuole. These are found typically developed among 

 the Ascidiidae (fig. 2, c, and PI. XXX. fig. 3, hi.). Other protoplasts secrete in their 

 interior pigment granules, generally of a dark-brown colour, and this process may be 

 carried to such an extent as to almost entirely obliterate the protoplasm of the cell 

 (fig. 2 (J., and PI. XXIX. fig. 3, p.c). Others, which remain in a less modified condi- 

 tion, are found scattered through the matrix in varying quantities and sizes. They may 

 be fusiform, rounded, stellate, or irregularly branched. 



The matrix is usually clear and homogeneous, with a gelatinous or cartilaginous 

 consistency. Frequently, however, it becomes fibrillated in parts, and in some cases, 

 especially amongst the Cynthiidae, is modified into a fibrous structure, very complicated 

 in the arrangement of its layers, and occasionally continued into simple or branched 

 spine-like projections from the outer surface. In other cases some parts of the test may 

 undergo a sort of cornification, so as to change their appearance and consistency. 



&&£&># ■■( 2 ■'-■■-■ ■ 



Fig. 2. — Transverse section through the test of Ascidia, showing the matrix in which lie large bladder cells (c) scattered in the inner 

 layers, and smaller bladder cells (a) near the surface (the left side of the figure), blood-vessels (6) with terminal knobs, and pigment 

 cells (d) — magnified about 40 times. 



Usually, especially when it is thick, the test is penetrated by a number of blood- 

 vessels, which are continued out from the body wall into the test, pushing a process of 

 the ectoderm before them. In the adult they enter as two large trunks placed close 

 together, usually near the posterior end of the ventral edge, and these two main stems 

 almost invariably give off corresponding branches which run together, ramifying chiefly 

 in the outer layers of the test (PI. XXIX., fig. 3), where they end in terminal bulbs, 

 usually by two twigs opening into one bulb, thus allowing the two vessels to communicate. 



Spicules have been described as occurring in the tests of various Tunicata. These are 

 probably in most cases post-mortem deposits, but in some species of Salpa siliceous 

 spicules appear to be normally present, while in certain of the Ascidise Composites, large 

 quantities of calcareous deposits are formed in the investing mass, and are in some eases 

 especially developed, as Giard has shown, in autumn, as a protection during the hiberna- 

 tion of the colony. 



