. Text-book of embryology. Embryology. 426 INVERTEBEATA CHAP. from the regions of the cuticular bands, and form the " wheel-organs " of the adult (Fig. 337, B). They eventually become connected with each other by a very narrow upper lip, which runs above the mouth and beneath the proboscis in such a manner that the latter is excluded from the trochal field. The dorsal feeler, or antenna, is formed at the spot where the first rudiment of the brain still retains a connection with the ectoderm. The muscles all appear to be of ectodermal origin. Those first observed by Zelinka were longi
. Text-book of embryology. Embryology. 426 INVERTEBEATA CHAP. from the regions of the cuticular bands, and form the " wheel-organs " of the adult (Fig. 337, B). They eventually become connected with each other by a very narrow upper lip, which runs above the mouth and beneath the proboscis in such a manner that the latter is excluded from the trochal field. The dorsal feeler, or antenna, is formed at the spot where the first rudiment of the brain still retains a connection with the ectoderm. The muscles all appear to be of ectodermal origin. Those first observed by Zelinka were longitudinal fibres running from the "wheel-organs" to the middle of the body. Circular muscles are. Fig. 337.—Ventral views of two embryos of Callidina russeola at a stage not long before hatching. (After Zelinka.) A, younger stage: stage of invagijijition of rudiments of wheel-organs. B, older stage: stage of evagination of rudiments of the wheel-organs and of preponderant growth of the tail, a, anus ; caAid, tail; caud,f, caudal fork; encfl, endodermal stomach; jyroh, rudiment of proboscis; stom, secondary stomodaeum ; tr, cuticular band marking place where cilia of the " wheels " will appear later. developed later from ectoderm cells, at regular intervals; these cells retain their plasma and their individuality, in contradistinction to most of the other ectoderm cells which flow together and form a thin syncytium, the so-called " ; The first traces of the excretory organs appear comparatively later as two streaks of dull glistening cells, situated at the sides of the pharynx. Subsequently they are developed into the coiled canal and adherent soleuocytes, of which in the adult there are eight on each side. The bladder into which they open is derived from the endoderm contained in the foot region; this endoderm has been pushed forward by the development of those ectoderm cells which constitute the foot Please note that these image
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