. Zoology : for students and general readers . Zoology. 264 ZOOLOGY. nation of the outer germ-layer ; the sides of the primitive mouth form the two sails of the velum or swimming organ, and the embryo now assumes the veliger stage (Fig. 192). Soon the middle germ-layer (mesoderm) arises, and from the cells composing it are developed the muscles of the foot and head, as well as the heart itself. The mantle or body- wall next develops, and from it the shell, which originates in a cup-like cavity which is connected only around the edge with the mantle, being free in the centre. The eyes and ears,
. Zoology : for students and general readers . Zoology. 264 ZOOLOGY. nation of the outer germ-layer ; the sides of the primitive mouth form the two sails of the velum or swimming organ, and the embryo now assumes the veliger stage (Fig. 192). Soon the middle germ-layer (mesoderm) arises, and from the cells composing it are developed the muscles of the foot and head, as well as the heart itself. The mantle or body- wall next develops, and from it the shell, which originates in a cup-like cavity which is connected only around the edge with the mantle, being free in the centre. The eyes and ears, or otocysts, next appear, both organs arising as an infolding of the outer germ-layer. Hitherto symmetrical, the alimen- tary canal now begins to curve to the left, and the visceral sac, or posterior part of the embryo hangs over on one side. The nervous system is the last to be developed. Fig. 193 represents the asymmetrical larva with the shell enveloping a large part of the body, and the ciliated velum {v) and foot (/) well developed. A temporary larval heart (li) assumes quite a different position from the heart of the adult, and the j)rimitive, deciduous kidney [h) is situated in quite a different place from the permanent kidney. The further changes consist in a gradual development of the hel- met-like shell, the disappearance of the temporary larval structures, and the perfection of the organs of adult life, the' gills appearing quite late. The development of Trochus, the top-shell, exhibits more strikingly the trochosf>here and veliger stages of molluscan life, and most Gastropods develop like this form. The velum at first forms a ciliated ring (Fig. 194, A, v) on the front end of the trochosphere. Fig. 194, B, represents the veliger state. It thus appears that the tem- porary larval or veliger form of the Gastropods are of vermian origin, the organs last to be de- veloped, i. e., the foot, shell and lingual ribbon, which are the distinctively molluscan characters, b
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectzoology, bookyear1879
