424 



ZOOLOGY 



SECT. 



(Fig. 350) fixes itself by means of the pre-oral lobe, which elongates 

 into a stalk (11), the cilia meanwhile being lost, and the apical 

 plate absorbed. The vestibule becomes closed, and a solid rudiment 

 of the adult oesophagus arises in close apposition with it. Round 

 .the oesophagus the hydrocoele grows in the form 

 of a ring. The vestibule (5) with the oesophagus 

 and hydroccele are rotated so as to come to lie 

 at the free extremity. The radial canals first 

 appear as five tentacles which at first project into 

 the cavity of the vestibule, and subsequently 

 when the latter opens out, as it soon does on 

 the exterior. The oesophagus (8), meanwhile, 

 has become completed, and the mouth pierces the 

 bottom of the now open vestibular cavity. The 

 arms appear as five processes which soon bifur- 

 cate : the five radial canals become applied to 

 them and undergo a corresponding division. 

 The first plates are formed while the larva 

 is still in the free condition ; in the fixed con- 

 dition they undergo further development, and 

 extend into the arms as they grow. After 

 about six months this pentacrinoid larva be- 

 comes free by the absorption of the stalk and 

 develops into the adult Antedon. 



In the transition from the bilateral 

 larva pluteus, bipinnaria, brachio- 

 laria, or auricularia to the radial 

 adult there is a marked metamorphosis. 

 As the adult form is developed on one 

 side of the larva, with its principal 

 axis at right angles to that of the 

 latter, the larval arms or processes 

 become absorbed. In the Holothu- 

 roidea and Ophiuroidea all the organs 



FiG.sso.-staiked larva of Antedon, of , the larva are carried on into the 

 from the right side ; calcareous adult \ in the Asteroidea and Echinoidea 



plates not represented. 1, right ,-1 -i ,-i 



co3iomic sac ; 2, enteric cavity ; 3, tne larval moutn and. ossopnagus are 



abolished and a new permanent mouth 

 and o^ophagus formed as a fresh in- 

 axial organ ; ii, fibrous strands in vagination from the surface. In the 

 (From Lang ' after very limited number of Echinoderms 

 which are viviparous there is no such 

 marked metamorphosis ; but even in these the larva is at first 

 distinctly bilateral in its symmetry. 



Ethology, etc. The Echinodermata are without exception l 

 inhabitants of the sea. In the adult condition the majority creep 

 1 One species of Synapta is said to inhabit brackish water. 



