ASCIDIANS 



apertures (Fig. 57, t'}. There is only a single layer of adult 

 ascidiozooids in the wall of the Pyrosoma colony, as all the fully 

 developed ascidiozooids are placed with their antero- posterior 

 axes at right angles to the surface and communicate by their 

 atrial apertures with the central cavity (Fig. 56, C). Their 

 dorsal surfaces are turned towards the open end of the colony, 

 and the buds are given off from their ventral edges (Fig. 57). 



Anatomy. The more important points in the structure of 

 the ascidiozooid of Pyrosoma are shown in Fig. 57. A circle of 



FIG. 57. Ascidiozooid of 

 Pyrosoma from the right 

 side, a, Anus ; A t, atrial 

 aperture ; at. in, .atrial 

 muscles ; Br, branchial 

 aperture ; br.s, branchial 

 sac ; d, cloaca ; d.l, dor- 

 sal lamina : d.t, dorsal 

 tubercle ; ec, ectoderm ; 

 en, endoderm ; end, endo- 

 style ; Ht, heart ; Lo, 

 luminous organ ; mes, 

 mass of mesoderm cells ; 

 m.f, muscle fibre ; n.g, 

 nerve-ganglion ; oes, oeso- 

 phagus ; sg, stigmata ; st, 

 stomach ; stol, stolon ; 

 t, test ; t', projection of 

 test near branchial aper- 

 ture ; tes, testis ; tn, ten- 

 tacle ; 1, 2, 3, buds. 



tentacles, of which one, placed ventrally (tn), is larger than the 

 rest, is found just inside the circular branchial aperture. From 

 this point a wide cavity, with a few circularly placed muscle- 

 bands running round its walls, leads back to the large branchial 

 sac (br.s.~), which occupies the greater part of the body. The 

 large stigmata are elongated transversely (dorso-ventrally), and 

 are crossed by internal longitudinal bars running antero-posteriorly. 

 The dorsal lamina is represented by a series of eight or ten 



