472 INVEKTEBKATA CHAP. 



columnar form. The ectoderm covering the brachiolar arms is 

 involuted into pockets, and these involuted portions are attacked 

 and devoured by phagocytes. In Asterina all the ectoderm of the 

 prae-oral lobe is disposed of in this way; but in Asterias, as the 

 prae-oral lobe shrinks in size, a good deal of the ectoderm which 

 originally covered it is drawn into the covering of the oral disc of 

 the star-fish. As the shrinkage of the prae-oral lobe goes on, the 

 sinuosities of the ciliated band, the "arms" of the larva, become 

 straightened out and thus obliterated. Finally, all that is left of 

 the prae-oral lobe is a small button projecting from the oral surface 

 of the star-fish (Fig. 362, C), but when the star-fish begins its free 

 life, as it wrenches itself free, the neck of this button is pulled out 

 into a long filamentous stalk, which eventually breaks through and 

 sets the star-fish free. 



DEVELOPMENT OF OTHER ASTEKOIDEA 



We may now glance at the peculiarities of the larvae of other 

 Asteroids whose life-history has been studied. Quite a number of 

 " species " of Bipinnaria are known, but few of them can be assigned 

 to any definite species of Asteroid. The complete life-cycle is only 

 known in the case of Asterias rubens, A. glacialis, and A. vulgaris. 



It has been doubted whether all Bipinnaria larvae develop 

 brachiolar arms, i.e. pass through a fixed stage. Gemmill has 

 recently shown that the larva of Porania pulvillus has such a stage ; 

 but extraordinary statements are made about the large Bipinnariae be- 

 longing to Asteroids of the family Astropectinidae, in which brachiolar 

 arms have never been observed. M. and C. Delap (1905) state that 

 in these larvae the star-fish rudiment is amputated from the posterior 

 half of the larva, the front half of which goes on living for a long 

 time after. Here is a matter which urgently requires reinvestigation. 



We may now turn to the consideration of the development of 

 Asterina gibbosa, which has been worked out by us in considerable 

 detail (MacBride, 1896). This development has already been alluded 

 to more than once. Its main peculiarities concern (a) the formation 

 of the larval gut and of the coelom, and (5) the external appearance 

 of the larva. 



With regard to (a) the first point, we find that Asterina gibbosa 

 has a prolonged embryonic life and only escapes from the egg-mem- 

 brane on the fourth day, when not only coelom, but also stomodaeum 

 and madreporic pore have been formed. The archenteron is spacious 

 and nearly fills the blastocoele. The coelom arises as an enormous 

 unpaired vesicle, constituting more than half the archenteron, and from 

 this vesicle prolongations, " tongues," extend backwards at the sides 

 of the gut (Fig. 363, B). Then transverse septa appear, which divide 

 off right and left posterior coeloms from an anterior unpaired coelom. 

 These septa are found in the " tongues " of the coelom, so that there 

 is an anterior portion of each tongue which belongs to the anterior 



