Ill CHARACTERS AND HABITS 65 



" tail," in which there is a skeletal axis, the urochord. A 

 relatively large cuticular test, the " house," may be formed with 

 great rapidity (in an hour or so) as a secretion from a part of the 

 ectoderm ; it is, however, merely a temporary structure which is 

 soon cast off and replaced by another. The branchial sac is simply 

 an enlarged pharynx with two ventral ciliated openings (stigmata) 

 leading to the exterior. These may be regarded as the repre- 

 sentatives of the primary gill-slits (undivided) of the Ascidian. 

 There are thus a single pair. There is no separate peribranchial, 

 atrial, or cloacal cavity. The nervous system consists of a large 

 dorsally placed ganglion and a long nerve-cord, which stretches 

 backwards over the alimentary canal to reach the tail, along 

 which it runs on the left side (morphological dorsal edge) of 

 the urochord. The anus opens ventrally on the surface of the 

 body, usually in front of the stigmata. No reproduction by 

 gemmation or metamorphosis is known in the life-history. 



Structure and Mode of Life. — This is one of the most 

 interesting groups of the Tunicata, as it shows more completely 

 than any of the rest the probable characters of the ancestral 

 forms. It has undergone little or no degeneration, and con- 

 sequently corresponds more nearly to the tailed, larval condition 

 than to the adult forms of the other groups. It retains, in fact, 

 the originally posterior, chordate, part of the body which is 

 lost in the metamorphosis of all the other Tunicata. Hence the 

 Appendicularians have been described as permanent, or sexually 

 mature, larval forms, and hence also the adult Ascidia may be 

 said to correspond to the trunk alone of the Appendicularian. 

 The Order includes a single group, the Appendiculaeiida, all 

 the members of which are minute (usually about 5 mm. in 

 total length) and free-swimming (Fig. 28). They occur near 

 the surface of the sea (and exceptionally in deeper water) in most 

 parts of the world, moving in a characteristic vibratory manner 

 by the contractions of the powerful tail (see Fig. 27). They 

 possess the power of forming with great rapidity, from tracts of 

 specially large glandular ectoderm cells, the " oikoplasts," an 

 enormously large (many times the size of the loody) investing 

 gelatinous layer, which probably corresponds to the test of other 

 groups, although it is doubtful whether it contains cellulose, and 

 it differs also in having no immigrated cells and in its temporary 

 nature. This structure (Fig. 28) was first described by Von 

 VOL. VII F 



