106 



ZOOLOGY. 



egg. When hatched, the larva is long, oval, and girded 

 with four zones of cilia, with a tuft of cilia at the end, a 

 mouth and anal-opening, and is about eight millimetres 

 long. The body-cavity is formed by an inversion of the 

 primitive layer which seems to correspond to the ectoderm. 

 Within a few hours or sometimes days, there are indica- 

 tions of the calcareous areolated plates forming the cup of 

 the future crinoid. Soon others appear forming a sort of 

 trellis-work of plates, and gradually build up the stalk, and 

 lastly appears the cribriform basal plate. Fig. 66, B, c, rep- 

 resents the young crinoid in the middle of the larva, whose- 

 body is somewhat compressed under the covering-glass. 



Fig. 67. Antedon, stalked and free. -From Macallister. 



Next appears a hollow sheath of parallel calcareous rods, 

 bound, as it were, in the centre by the calcareous plates. 

 This stalk (B, c) arises on one side of the digestive cavity 

 of the larva, and there is no connection between the body- 

 cavity of the larva and that of the embryo crinoid. 



Two or three days after the appearance of the plates of 

 the crinoid, the larva begins to change its form. The 

 mouth and digestive cavity disappear, not being converted 

 into those of the crinoid. The larva sinks to the bottom, 

 there resting on a sea- weed or stone, to which it finally ad- 

 heres. The Pentacrinus form is embedded in the larval body 



