42 ASCIDIANS CHAP. 
loop gives rise to the double or paired “ vessels” seen branching 
through the test (see Fig. 17, v). The two vessels of a pair are 
one blood-channel imperfectly divided by a connective-tissue 
septum. The blood courses out along one side, round the com- 
munication in a “terminal knob” at the end, and back down the 
other side. The “terminal knobs” are very numerous, and form 
a marked feature in the outer layer of the test (Fig. 17, tk); 
in some cases (Culeolus murrayt), they probably form an accessory 
organ of respiration, while in others (Botryllidae), they pulsate 
and aid in keeping up the circulation. 
The ectoderm is a simple epithelial layer (Fig. 16, ec). It is 
turned in for a short distance at the branchial aperture (mouth), 
Dates IW 7/e 
Section through the surface layer of test of Ascidia mentula, x 50. 6. Bladder 
cells ; ¢.c, test cell ; ¢.4, terminal knobs of vessels ; v, vessels of test. 
and atrial aperture (cloaca), as a short stomodaeum and_procto- 
daeum respectively, lined in each case by a delicate prolongation 
of the test. 
Fig. 24, A, p. 52, shows the relations of ectoderm, mesoderm, 
and endoderm in a section through the antero-dorsal part of the 
body. The cavity marked p.br is a portion of the atrial cavity 
lined by ectoderm, and must not be confounded with a coelom. 
The absence of a true coelom in the mesoderm will be noticed 
in this and other figures, and yet the Tunicata are Coelomata, 
although it is very doubtful whether the enterocoel which has 
been described in the development of some is ever found. The 
coelom is in any case largely suppressed later, and is only re- 
presented in the adult by the pericardium and by small cavities 
in the renal and reproductive organs and ducts. 
Body-Wall and Cavities of the Body.—The name “ mantle ” 
is given to the ectoderm with the parietal mesoderm which form 
the body-wall inside the test. It is largely formed of connective 
