REPORT ON THE TTJNICATA. 281 



other foreign bodies, so as to form a continuous, more or less solid, protecting, and 

 concealing coat. This property is characteristic of the Molgulidae, hut we find it more 

 rarely, though just as well developed, in other families, as, for example, in the case of 

 Polycarpa molguloides (PI. XXII. fig. 5) in the Cynthiidae, and Ascidia conchilega in 

 the Ascidiidae. 



In other cases, again, we find the protective covering of sand adhering directly to the 

 test, no hair-like processes having been developed. Under these circumstances, the coat ing 

 is usually more or less imperfect, and rarely attains any great thickness ; in Polycarpa 

 tinctor (PI. XXI. figs. 1-3), however, there is a complete layer of sand over the outer 

 surface of the test. 



Calcareous spicules are present in the internal tissues of two genera of Simple Ascidians, 

 viz., Culeolus and Cynthia, and in each case they are found in several species of the 

 genus. They are, however, very different in their character in the two genera, being 

 irregularly branched, and with smooth surfaces in Culeolus (Plates VIII. to XIII.), 

 while they are simply rod-shaped or fusiform in Cynthia, but have their surfaces minutely 

 echinated (Plate XVII.). 



In Culeolus they are present in all the species except Culeolus perlucidus, chiefly in 

 the walls of the endostyle and the branchial sac, but also in the tentacles, the dorsal 

 lamina, and other organs. But though varying in details, such as size, stoutness, number, 

 and length of branches, &c, according to the species and organ, still they have throughout 

 the genus, wherever found, a very great similarity. They have a characteristic appearance 

 which is difficult to describe, but which seems to depend upon their irregular, but 

 rather graceful, mode of branching, their invariably rounded angles and curved ends, 

 and lastly, upon the system of concentric markings, like contour lines, in their interior 

 (PI. VIII. fig. 6). 



In the genus Cynthia, on the other hand, the calcareous spicules have a totally different 

 appearance. They have been found in three species, namely, Cynthia pallida, Heller, 

 where they were first discovered, Cynthia convplanata, Herdman, and Cynthia 

 papietensis, n. sp., and they occur in the test and the mantle, but especially in the 

 vessels of the branchial sac. 



Just as in the case of the spicules of Culeolus, so also in Cynthia, the spicules, though 

 differing in various details according to the species and the part of the body in which 

 they are found, still have throughout a very characteristic appearance. They are always 

 elongated in shape, sometimes cylindrical, often more or less fusiform, but they are never 

 branched (PI. XVII. figs. 3, 8, 13, 15, and 19). The outer surface is always orna- 

 mented with usually closely-placed transverse rows of very minute spine-like processes, 

 which give a most characteristic appearance to the spicule, and render it very easily 

 recognisable in any tissue (PI. XVII. figs. 5, 6, and 20). 



In all cases, in both Culeolus and CynlJii", these spicules appear to be formed in the 



