. Elementary text-book of zoology. Zoology. GEPHTRBA. 391 posterior segmental organs (anal vesicles, fig. -'314, Ab) in the terminal segment, each of which has a immber of peritoneal funnels and opens into the rectum. In Bonellia the segmental organ which performs the function of uterus is, like the ovary, single (tig. 314 ft). Development.—The development of the ovum begins with an unequal segmentation. In Bonellidi the small cells of the animal pole grow round the four large yolk spheres, which give rise to the entoderm, leaving a small aperture, the blastopore (fig. 110). The Ec
. Elementary text-book of zoology. Zoology. GEPHTRBA. 391 posterior segmental organs (anal vesicles, fig. -'314, Ab) in the terminal segment, each of which has a immber of peritoneal funnels and opens into the rectum. In Bonellia the segmental organ which performs the function of uterus is, like the ovary, single (tig. 314 ft). Development.—The development of the ovum begins with an unequal segmentation. In Bonellidi the small cells of the animal pole grow round the four large yolk spheres, which give rise to the entoderm, leaving a small aperture, the blastopore (fig. 110). The Ecldnrus larva- (iig. 315) are the most accurately known. They present the type of Loven's larva and possess a strongly developed. FIG. 315.—n, Larva of Echiui-us from the ventral side (after Hatschek). SP, apical plate; Prw, prreoral circle of cilia ; Pow, postoral circle of cilia; En, head-kidney ; Vg, ventral ganglionic cord connected with the apical plate by the long- cesophageal commissures ; AS, anal vesicle. l>, Ventral region of the Echlurus larva with segmented mesodermal hands; SC, oesophageal commissure ; Dap, dissepiments of the anterior body segments; MS, mesodermal bands ; A, anus. prseoral circle of cilia (Prw), in addition to which there is also a delicate post-oral circle of cilia (Pow). Early in larval life a seg- mental organ, the head kidney or pi'onephros (A'-Vr), is developed, one on either side; and behind'it a pair of mesoblastic bands makes its appearance and gives rise in the subsequent development to the rudiments of 15 segments (fig. 315 b). In the terminal segment, which is surrounded by a circle of cilia, there appear segmeutal. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Claus, Carl, 1835-1899; Sedgwick, Adam, 1854-1913; Sinclair, F. G. (Frederick Granville), b. 1858.
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
