" tniiicine," a substance which appears to be identical 

 in composition, and in behaviour under various treatments, 

 v^ith "cellulose" — a characteristically vegetable substance, 

 entering largely into the composition of plants. The test 

 is a protective layer, usually about a quarter of an inch 

 in thickness, and may be considered as an exo-skeleton. 

 It is the only tissue of a skeletal nature which the adult 

 Ascidian possesses. 



In shape the test is an oblong sac, pierced only by the 

 branchial and atrial apertures, and forming, at its posterior 

 end and left side, the place of attachment by which the 

 Ascidian adheres to the rock (PI. II., figs. 1 and 4). At 

 this point it may become greatly thickened and expanded 

 to form a margin, or may even grow out in the form of a 

 short stalk, raising the body above the surrounding surface. 

 Stones, sea -weeds, dead shells, and remains of other 

 animals may be overgrown by the test and incorporated 

 in its substance ; many sessile animals and various kinds 

 of sea-weeds may become attached to its outside ; and 

 some parasitic Amphipod Crustaceans (Tritceta gihhosa) 

 and Lamellibranch Molluscs (Modiolaria marmorata) may 

 inhabit cavities excavated in its thickness. Microscopic 

 parasitic Algae may also be present amongst the cells in 

 the interior of the test, and help in giving the animal its 

 colour. 



The test is cartilaginous in appearance and consistence, 

 and to some extent in structure, as it consists of a clear 

 or slightly fibrillated matrix in which are imbedded many 

 cells. As the test is morphologically a cuticle, being at 

 first a secretion on the outer surface of the ectoderm (PI. 

 II., fig. 5, ec), the cells it contains have immigrated to it 

 from the body ; and it has been shown by Kowalevsky 

 and others that many of these are mesodermal cells or 

 wandering amoebocytes which have passed through the 



