oT 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-shts (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 APPENDICULARIIDA, 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 body) 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 
