THE STRUCTUEE AND AFFINITIES OF THE SEA-SQUIRTS. 241 
which have no popular designation, but are known to naturalists 
as the genera Salpidce and Pyrosomidce. 
Before, however, proceeding to survey the almost Protean 
variations which the Timicata present, we will examine, some- 
what in detail, the structure of a representative, both of the 
nomad tribes, and of those which have a fixed habitat, the lease 
of which expires only with life. 
To begin with the latter. The outer coating of the Ascidian, 
which varies in consistence, is termed the test.” * This is 
pierced by two openings, situated at a varying distance from 
one another, sometimes occupying the extremity of two necks, 
which render the creature a caricature of the Wolff ” bottle, 
known to workers in the laboratory. One of the two orifices (the 
uppermost, if they be on a different level) admits water (air- 
laden) and food ; the other gives exit to excretions, intestinal 
and generative, and to water (airless). (Figs. 1 and 10, T.) 
A most important point in connection with the test is that it 
contains cellulose (no less than 60 per cent.), the possession of 
which substance was once held to be the exclusive privilege of 
the vegetable world. Its presence, however, is supposed to 
be due to a destructive rather than to a constructive chemical 
change. 
It is significant that the fresh -water polype, or Hydra^ con- 
tains chlorophyll, and this, moreover, not derived from vege- 
table food, but elaborated in its own tissues. The test is the 
true homologue of the shell of bivalves — e.g., the river-mussel 
(Anodonta Cygnea). 
Beneath the test lie two inner coats — namely, the muscular 
‘‘inner tunic” (Hancock), which is homologous with the “ man- 
tle ” of bivalves ; and the “ lining membrane ” ( “ inner tunic,” 
Huxley). These two tunics are generally closely adherent, ex- 
cept where the viscera and blood-channels intervene. (Figs. 1 
and 10, M.) 
The mantle and test are usually free (this is very evident in 
spirit -preserved specimens), except at the two respiratory 
orifices mentioned above, where they are adherent, and at points 
where vessels pass to the test, serving as slings or side-stays. 
The bag formed by the combined mantle and lining mem- 
brane copies in its outline that of the inner surface of the test ; 
and has, moreover, two orifices corresponding with those pierced 
in the latter. 
A little distance within the passage of entrance (“ inhalent ”) 
into this bag is a circlet of tentacles, pointing downwards and 
* From the Latin testa , a piece of baked clay, a potsherd, a wine-jar. 
“ Quo semel est imbuta recens servabit odorem 
Testa dill.” — Har. 
