514 THE ANATOMY OF INVERTEBRATED ANIMALS. 



It is obviously the homologue of the test of other Ascidians, 

 which is often adherent to the ectoderm by only two or three 

 points ; but no cellulose has been discovered in it. Accord- 

 ing to M. FoJ, who has studied the formation of the " house " 

 with great care, the Appendicularioe have no proper test, and 

 what I have described as the structureless gelatinous invest- 

 ment of the anterior part of the body is the commencement 

 of the " house." It increases, assumes a peculiar fibrous 

 structure, and in the course of an hour, in a vigorous animal, 

 it is separated as an envelope in which the whole body is 

 capable of free movement. In front, it presents two funnel- 

 shaped apertures supported by a fibrous trellis-work, which 

 lead down to the cavity in which the body is contained. A 

 spacious median chamber allows of the free motion of the tail. 

 After a few hours the animal deserts its test and forms an- 

 other. 



In the great majority of those Tunicata which are fixed 

 in the adult state, the young leave the egg in an active lar- 

 val condition, and resemble Appendicularia in being pro- 

 pelled by a muscular appendage in the axis of which lies an 

 urochord. The body and appendage, however, are invested by 

 a coat, or test, impregnated with cellulose, and the former 

 presents some important structural differences from that of 

 Appendicularia. After a free existence of a certain dura- 

 tion, the body of the larva fixes itself, the appendage withers 

 away, and the young animal assumes the ordinary form of a 

 fixed Ascidian. It may remain simple, or it may develop 

 buds and give rise to a compound organism or Ascidiarium, 

 consisting of many Ascidiozooids united together. 



All the fixed Tunicates present two, more or less closely 

 approximated, apertures : one, oral, leads into the alimentary 

 cavity ; the other, atrial, opens into a chamber, the atrium, 

 into which the faeces and genital products are poured. During 

 life, when these apertures are open, a current sets into the 

 oral and out of the atrial opening. But if the animal is irri- 

 tated, the sudden contraction of the muscular walls of its 

 body causes the water contained in the brachial and atrial 

 cavities to squirt out in two jets, while both apertures are 

 speedily closed. 



The apertures are much farther apart in some forms than 

 in others, and in certain of the Botryllidce they are almost 

 terminal. In the pelagic genera Pyrosoma (Fig. 150), Dolio* 

 lum (Fig. 151), and Salpa (Fig. 152), the atrial and oral aper- 



