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 lies, and dis- 

 tributes it to the ascidio- 

 B zooids by means of a heart 

 and system of vessels which 

 I have been meanwhile 

 formed. When the cyatho- 

 zooid atrophies and is 



Fig. 58.— Development of Pyrosnma colony. A, absorbed, its Original atrial 



young stage showing oozooid or cyathozooid, aperture remains and 



cy, "With stolon divided into tour blastozooids ^ 



(i.-iv.) : V, viteiius. B, older stage showing deepens to become the cen- 



tlie four blastozooids in a ring around the . i ' ^. l r j-i, 



remains of the cyathozooid. (Alter Salensky.) ^^1 Cavity of the yOUng 



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 fom" 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, 

 Peron, and P. giganteum, 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. elegajis, 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. spinosiom, Herdman, was found by the 

 " Challenger" in both North and South Atlantic in 1873 ; and 



^ According to Kowalevsky. Salensky, however, considers that the atrial 

 aperture closes, and that a new surface depression appears later. 



