. Text-book of embryology. Embryology. 344 INVEETEBEATA CHAP. to a horizontal position, and the loop of the intestine is straightened out. After metamorphosis the labial palps, so characteristic of the adult mussel, are formed in an amazing manner. The cerebral pit, from which the rudiments of the cerebral ganglia have been separated, flattens out and forms two lateral bands of ciliated epithelium above and at the sides of the mouth. These develop into the upper and outer labial palps, while downgrowths from their inner ends give rise to the inner and lower labial palps (Fig. 276). The shell b

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. Text-book of embryology. Embryology. 344 INVEETEBEATA CHAP. to a horizontal position, and the loop of the intestine is straightened out. After metamorphosis the labial palps, so characteristic of the adult mussel, are formed in an amazing manner. The cerebral pit, from which the rudiments of the cerebral ganglia have been separated, flattens out and forms two lateral bands of ciliated epithelium above and at the sides of the mouth. These develop into the upper and outer labial palps, while downgrowths from their inner ends give rise to the inner and lower labial palps (Fig. 276). The shell begins to alter its larval shape by a preponderant growth of its posterior and lower angle, and by a certain amount of growth. Fig. 274.—Horizontal .section through the anterior portion of an old Veliger of DreUsensia polymorpha in order to show the differentiation of the cerebral ganglia from the cerebral pit. (After Meisenheimer.) eg, cerebral ganglion ; c.-p, cerebral pit; oes, oe.sopl)agu.s ; pl.g, rudiment of pleural ganglia ; sh, sbell; V, velum. of its anterior angle also. The gill papillae grow into long filaments, which become locked together by the longer cilia on their lateral faces. The development of an additional outer row of gill filaments, and the bending up of the ends of the filaments to form the reflected portions of the gill lamellae, take place later in life (Fig. 275). The rudiments of the kidneys acquire cavities. The ring of cells surrounding the intestine becomes double and the two layers separate from one another, the cavity between them being the pericardium, i.e. the coelom. The cavity of the heart is the space between the inner of these layers and the intestine. The kidney becomes U-shaped, and the inner limb of the U on either side (which will form the ureter) coalesces with the corresponding part of the other kidney so as to form a transverse space beneath the intestine. Into this space opens, on either side, a diverticulum of the mantle-cavity,