lis THE VOYAGE OF H.M.S. CHALLENGER. 



The Test (Testa, Tunica Externa, Cellulose Mantle, Outer Mantle, Cartilaginous Sac). 



The test or outer tunic (sometimes incorrectly called the mantle), from which the 

 class derives its name, is one of the most characteristic features of the group. It under- 

 goes many and various modifications, but it is present, and is recognisable as a " test," 

 or " investing mass," in almost every member of the class. It possesses the additional 

 interest of being the organ in which Karl Schmidt, in 1845, made his well-known 

 discovery of cellulose in the animal kingdom. Since that date the principal works on 

 the structure of the test in the Tunicata have been those of Lowig and Kolliker (1846), 

 Schacht (1851), F. E. Schulze (1863), Oscar Hertwig (1872), and Semper (1875). 



In a Simple Ascidian the test is a more or less thickened coat forming the outer layer 

 of the body wall. It varies greatly in shape according to the species, and even to a 

 considerable extent according to the individual, being greatly affected by surrounding 

 circumstances. In the so-called Social Ascidians (the family Clavelinidse) it forms in 

 addition the stolons or creeping roots, which connect the different individuals of the 

 " Society," and contain the connecting blood-vessels. In the Ascidise Compositse it is 

 represented by the " investing mass," the tests of the different Ascidiozooids of the colony 

 having fused together into a common ground mass or matrix. In Pyrosoma also there 

 is a colonial mass representing the united tests of the different Ascidiozooids ; while 

 in Salpa the test forms the outer layer of the body of each individual, as in the Ascidia? 

 Simplices. In the Appendiculariidae the test is represented by the structure usually 

 known under the name " Haus," given by its discoverer Mertens. This is only formed 

 at certain times, so that in these Tunicates there is sometimes no test. In certain species 

 of Doliolum, also, there appears to be no test. 



The external surface is always more or less irregular and prone to develope knobs, 

 hairs, spines, and other processes. This is especially the case at the posterior end of the 

 fixed forms, where the test frequently pushes out long branched processes to serve 

 as roots for the attachment of the Ascidian to foreign bodies. 



In the adult condition it is histologically an abnormal form of connective tissue, con- 

 sisting of protoplasts of various forms imbedded in a matrix which also varies greatly in 

 its characters. The test is in all cases lined by a layer of epithelial cells, the true ectoderm, 

 and it has its origin from these cells alone. It commences as a cuticular secretion on the 

 surface of the ectoderm, and afterwards attains its cellular condition by the migration 

 into it of protoplasts formed by proliferation from the ectoderm. These immigrated cells 

 then increase the thickness of the test greatly by forming deposits of cuticular matter 

 around themselves, thus building up the matrix of the adult test. The protoplasts may also 

 develop large vacuoles in their interior, which sometimes increase to such an extent as to 

 form what may be called a bladder-cell — a large oval or globular space in the matrix lined 



