. The Cambridge natural history. Zoology. 94 ASCIDIANS 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 Pyr
. The Cambridge natural history. Zoology. 94 ASCIDIANS 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 ^ (): 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 Germ
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
