. Text-book of embryology. Embryology. VIII AETHROPODA 243 takes place in connection with a small invagination which occurs at one pole. From the lips of this invagination at any rate the meso- derm is developed. It would appear that the early development of Pallene hears some considerable resemblance to that of Palaemon (see p. 192). Pallene emerges from the egg when it has almost attained the adult condi- tion, but most Pantopoda emerge as larvae with three pairs of legs and pursue a semiparasitic life inside Hydroid polyps, gradually attaining the adult condition after a series of moults, a
. Text-book of embryology. Embryology. VIII AETHROPODA 243 takes place in connection with a small invagination which occurs at one pole. From the lips of this invagination at any rate the meso- derm is developed. It would appear that the early development of Pallene hears some considerable resemblance to that of Palaemon (see p. 192). Pallene emerges from the egg when it has almost attained the adult condi- tion, but most Pantopoda emerge as larvae with three pairs of legs and pursue a semiparasitic life inside Hydroid polyps, gradually attaining the adult condition after a series of moults, at each of which a new pair of legs is developed. TAEDIGKADA A word or two may here be interposed about the development of the Tardigrada, though it is exceedingly doubtful whether these minute, degenerate Arthropoda are really related more closely to Arachnida than to the other groups. They possess no jaws or antennae and only four pairs of stumpy unbranched appendages. The development of one species, MacTohiotus macronyx, has been worked out by Erlanger (1895). He asserts that the minute egg undergoes total segmentation, and that a hollow blastula is formed from which a gastrula arises by invagination. From the arch- enteron four pairs of coelomic sacs arise as hoEow outgrowths, and there is also a median posterior sac arising in the same way, from which genital organs and Malpighian tubules arise. It is possible, however, that Erlanger's account of the de- velopment of the coelom is incorrect, as it was founded on whole mounts and not upon ffon FlQ. 192.—Dorsal view (optical frontal sec- tion) of embryo of Macrobiotus macronyx. (After Erlanger.) coei'Sj coelomic sacs, of which the first and the last can be seen to open into the gut; gon, terminal coelomic sac, which is the rudiment of the gonad ; oes, oesophagus; , eotoderuiio thickening, rudiment of supra - oesophageal ganglion ; sf, stomach ; stowi, rudiment of the stomodaeum. ANCESTRAL HISTORY OF THE AEAC
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