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 

 sections. 



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 ; s.o.g, eotoderuiio thickening, 

 rudiment of supra - oesophageal ganglion ; sf, 

 stomach ; stowi, rudiment of the stomodaeum. 



ANCESTRAL HISTORY OF THE AEACHNIDA 



When we survey the development of Arachnida so far as it is 

 known we are struck by a fundamental agreement in type in animals 

 so diverse as Limulus, a scorpion, and a spider. In all of them the 

 yolk is so abundant that no trace of a complete segmentation ol the 



