42 



ASCIDIANS 



loop gives rise to the double or paired " vessels " seen branching 

 through the test (see Fig. 1 7, 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. Tlie " terminal knobs " are very numerous, and form 

 a marked feature in the outer layer of the test (Fig. 17, t.k); 

 in some cases (Culeolus murrayi), 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), 



11. 



Fig. 17. — Section through the surface layer of test of Ascidia mentula, x 50. bt. Bladder 

 cells ; t.c, test cell ; t.k, 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 2^.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 reproduc|.ive 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 



