. Elementary text-book of zoology. Zoology. of the stomach and the circular vessel, which is formed from the dorsal tube. They are at first enclosed in a cavity, from which later on they penetrate to the exterior. The larva retracts its lateral lobes and transforms itself into a barrel- shaped body with five transverse rows of cilia, and loses the mouth and dorsal pore (fig. 229). The ambulacral system gradually de- velops further, the intestine be- comes longer, the first five tentacles break through to the exterior, the mouth appears at the anterior pole, and the first suctorial foot with it
. Elementary text-book of zoology. Zoology. of the stomach and the circular vessel, which is formed from the dorsal tube. They are at first enclosed in a cavity, from which later on they penetrate to the exterior. The larva retracts its lateral lobes and transforms itself into a barrel- shaped body with five transverse rows of cilia, and loses the mouth and dorsal pore (fig. 229). The ambulacral system gradually de- velops further, the intestine be- comes longer, the first five tentacles break through to the exterior, the mouth appears at the anterior pole, and the first suctorial foot with its ambulacral vessel is seen on the ventral surface (fig. 230). The animal gradually loses the bands of cilia, and as a young Holothurian creeps about by means of its ten- tacles and of the first ambulacral foot, which is soon followed by a second new one. In the more direct development the bilateral larval form seems to be completely suppressed, and the time of free-swimming life shortened or altogether dispensed with. In these cases, protective arrangements, such as brood pouches, are always present in the mother. The brood pouch of Pteraster militaris is the most carefully pro- tected. It lies above the anus and generative open- ings ; its walls are highly charged with calcareous matter, and they are raised above the spicules on the back. From eight to twenty ova (1 mm. in diameter) pass into the interior of the brood pouch, and are there developed into oval embryos, which acquire several sucking feet and assume later the form of a star with five rays. •E FIG. 229.—Auricularia pupa of seen in profile (after E. Metschnikoff). The mouth is already large, so that the tentacles (T) can be protruded. Wr, Ring of cilia; Pe, Pi, somatic and visceral layers of the peritoneal sacs; Ob, auditory vesicle; Po, pore of the water-vascular system; R, cal- careous wheel-shaped FIG. 230.—Young Holothurid with extended ten- tacles (T), swimming and creeping (after J. Miiller).. Pl
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
