94 ASCIDIANS CHAP, 
completed its development, but having precociously given rise to 
the budding stolon. 
As the four ascidiozooids increase in size, they grow round 
the cyathozooid and soon encircle it (Fig. 58, B). In this con- 
dition the young colony leaves the body of the parent and becomes 
free. The cyathozooid ab- 
sorbs the nourishing yolk 
upon which it les, and dis- 
tributes it to the ascidio- 
zooids by means of a heart 
and system of vessels which 
have been meanwhile 
formed. When the cyatho- 
zooid atrophies and _ is 
Fia. 58.—Development of Pyrosoma colony. A, absorbed, its original atrial 
young stage showing oozooid or ecyathozooid, , aes 
cy, With stolon divided into four blastozooids aperture remains and 
(1.-IV.): v, vitellus. B, older stage showing deepens to become the cen- 
the four blastozooids in a ring around the Se sea ee ye 
tral cavity” of the young 
remains of the cyathozooid. (After Salensky.) 
colony, which now consists 
of four ascidiozooids placed in a ring, around where the cyatho- 
zooid was, and enveloped in a common test. The test is at first 
formed by the ectoderm cells of the cyathozooid. Later it 
becomes invaded by mesoblast cells from the ascidiozooids in the 
usual manner. The colony gradually increases by the formation 
of buds from these four original ascidiozooids. The young colony 
is, in some species, at first male, and only becomes hermaphrodite 
when it has attained to some size. 
Occurrence.—The half-dozen known species of Pyrosoma 
are widely distributed over the great oceans, although they are 
probably most abundant in tropical waters. Pyrosoma atlanticum, 
Péron, and P. gigantewm, Lesueur, are the commonest forms. 
Although sometimes abundant in the Mediterranean and the North 
Atlantic they have apparently not been found in British seas. 
P. elegans, Lesueur, is a Mediterranean form allied to the last two ; 
and P. minatum and P. aherniosum, Seeliger, were discovered 
during the German “ Plankton ” expedition in the tropical Atlantic. 
Finally, the enormous P. spinoswm, Herdman, was found by the 
“ Challenger” in both North and South Atlantic in 1873; and 
1 According to Kowalevsky. Salensky, however, considers that the atrial 
aperture closes, and that a new surface depression appears later. 
