212 ZOOLOGY. 



ways : ( i ) by migrating cells budded from the entoderm into 

 the segmentation cavity (mesenchyma; Fig. 12, c) ; and (2) 

 by the outgrowth of ccelomic vesicles or pouches from the wall 

 of the archenteron or entoderm (true mesoderm). These 

 latter outpockets of the wall of the gut are those which give 

 rise to the coelom and to the water vascular system (see 



241). 



In the later larval development the cilia of the gastrula 

 become limited to two zones, a preoral and a preanal, 

 and the shape of the larva is much modified. Numerous 

 paired, lateral outgrowths serve to accentuate the funda- 

 mental bilateral symmetry. In most members of the group 

 a marked metamorphosis occurs in the passage from the larval 

 to the adult condition. During this change, the water vascular 

 system and the mid-gut of the larva are retained with the 

 necessary modifications. About these as a centre, what we 

 might almost call a new animal, the radiate star-fish, begins 

 to grow at the expense of the larval organs which are ab- 

 sorbed by the amoeboid cells, and thus new organs appropriate 

 to the adult are formed. During this process the bilateral 

 symmetry of the embryo gives place to the radial symmetry 

 of the adult. While there is no reproduction by budding 

 there is a striking power of renewal of arms or other portions 

 which may be lost by injury, or in some instances by self- 

 mutilation. ^.rms are readily reproduced if the disc is un- 

 injured (stars, brittle-stars, and crinoids) ; portions of the 

 internal organs, as the digestive tract, are said to be regen- 

 erated by some of the holothurians. Occasionally, at least, 



* 



an arm seems to have the power of reproducing a new disc 

 and other arms. This power of throwing off arms and re- 

 placing them is doubtless a means of defense. 



249. Ecology. The echinoderms are marine. The larvae 

 are free-swimming, pelagic, but after the assumption of 

 the adult form they usually become much less active. The 

 crinoids are typically stalked and often attached. The 

 asteroids and echinoids inhabit the bottom of the ocean where 



