. Bulletin - United States National Museum. Science. the so-called cyathozooid, corresponds to the solitary form of Doliolum and of Salpa (fig. 7). In Pyrosoma the solitary generation is reduced to a transient, embryonic stage. This cyathozooid gives rise very early to four buds, or primary ascidiozooids, which in turn bud and form a great number ol secondary ascidiozooids; and thus the colony is established. The stolon of the cyathozooid develops very early as a prominent, sac-like outgrowth from that end of the embryo which lies opposite to the cloacal aperture (figs. 6 and 7). This outgrowt
. Bulletin - United States National Museum. Science. the so-called cyathozooid, corresponds to the solitary form of Doliolum and of Salpa (fig. 7). In Pyrosoma the solitary generation is reduced to a transient, embryonic stage. This cyathozooid gives rise very early to four buds, or primary ascidiozooids, which in turn bud and form a great number ol secondary ascidiozooids; and thus the colony is established. The stolon of the cyathozooid develops very early as a prominent, sac-like outgrowth from that end of the embryo which lies opposite to the cloacal aperture (figs. 6 and 7). This outgrowth soon becomes marked off into four short segments, by partial constrictions. Each of these segments is destined to develop into a primary ascidiozooid. The stolon, originally straight, becomes curved to the left, so that by fur- ther growth it comes to encircle equatorially the yolk of the cyathozooid. The buds all lie with their ventral surfaces distal from the parent. The earliest organs of the chain of buds origi- nate for the most part as direct continuations of the corresponding organ rudi- ments in the cyathozooid. The ectoderm is directly continuous in the two. The enteric canal is a tu- bular outgrowth running throughout the chain. There is found in the cy- athozooid the rudiment of an endostyle-fold on the outer layer of endoderm, and this also is prolonged into the stolon. In addition to these, are a pair of peribranchial tubes, a transient pericardial tube on the right side of the body, and two strands of mesoblast cells. The nervous system has a new origin in each bud, forming as an invagination of the dorsal ectoderm. During the growth of its buds the ectoderm of the cyathozooid secretes a layer of cellulose around itself and its buds, just within the A. ,-777. Fig. 7.—Diagrams to snow tue position of tue organs in a Pyrosoma cyathozooid (A) and a Salpa (£), SOLITARY FORM. AFTER KoRSCHELT AND HEIDER (1900).. Please note that these images are extracted from scan
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Keywords: ., bookauthorun, bookcentury1800, bookdecade1870, booksubjectscience
