Fig. 248. Three stages in the development of Holothuria tubulosa viewed from the side in optical section. (After Selenka.)
m. mouth; oe. œsophagus; st. stomach; i. intestine; a anus; l.c. longitudinal ciliated band; v.p. vaso-peritoneal vesicle; p.v. peritoneal vesicle; p.r. right peritoneal vesicle; pl. left peritoneal vesicle; w.v. water-vascular vesicle; p. dorsal pore of water-vascular system; ms. muscle cells.
The vaso-peritoneal vesicle undergoes a series of remarkable changes. After its separation from the archenteron it takes up a position on the left side of this, elongates in an antero-posterior direction, and from about its middle sends a narrow diverticulum towards the dorsal surface of the body, where an opening to the exterior becomes formed ([fig. 248] B, p.). The diverticulum becomes the madreporic canal, and the opening the dorsal pore.
The vaso-peritoneal vesicle next divides into two, an anterior vesicle ([fig. 248] B, w.v.), from which is derived the epithelium of the water-vascular system, and a posterior ([fig. 248] B, p.v.), which gives rise to the epithelioid lining of the body cavity. The anterior vesicle ([fig. 248] C, w.v.) becomes five-lobed, takes a horseshoe-shaped form, and grows round the œsophagus ([fig. 256], w.v.r). The five lobes form the rudiments of the water-vascular prolongations into the tentacles. The remaining parts of the water-vascular system are also developed as outgrowths of the original vesicle. Five of these, alternating with the original diverticula, form the five ambulacral canals, from which diverticula are produced into the ambulacral feet; a sixth gives rise to the Polian vesicle. The remaining parts of the original vesicle form the water-vascular ring.
We must suppose that eventually the madreporic canal loses its connection with the exterior so as to hang loosely in the interior, though the steps of this process do not appear to have been made out.
Fig. 249. Longitudinal section through an embryo of Cucumaria doliolum at the end of the fourth day.
Vpv. vaso-peritoneal vesicle; ME. mesenteron; Blp., Ptd. blastopore, proctodæum.