IX 



PHYLUM ECHINODERMATA 



433 



ant 



with a tuft of sensory cilia. The vibratile cilia on the surface are 

 arranged in five transverse bands (I-V). Between the second and 

 third of these is a wide shallow depression, the vestibule or 

 stomodseum (1), which does not communicate with the mouth. 

 After remaining in the free condition 

 for a short time, the larva (Fig. 345) 

 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, arid 

 a solid rudiment of the adult 

 oesophagus arises in close apposition 

 with it. Round the oesophagus the 

 hydroccele 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 bifurcate : the five radial canals 

 become applied to them and un- 

 dergo a corresponding division. The 

 first plates are formed while the 

 larva is still in the free condition ; in 

 the fixed condition they undergo 

 further development, and extend into 

 the arms as they grow. After about 

 six months this pentacrinoid larva 

 becomes free by the absorption of 

 the stalk and develops into the adult 

 Antedon. 



In the transition from the bi- 

 lateral larva pluteus, bipinnaria, 

 brachiolaria, 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 

 Holothuroidea and Ophiuroidea all the organs of the larva are 



VOL. I F F 



post 



FIG. 345. Stalked larva of Antedon, 

 from the right side ; calcareous 

 plates not represented. 1, right 

 ciBlomic sac ; 2, enteric cavity ; 3, 

 left coelomic sac ; 4, sacculi ; 5, 

 vestibule, still closed ; 6, primary 

 tentacles ; 7, secondary tentacles ; 

 8, oesophagus ; 9, rectum ; 10, axial 

 organ; 11, fibrous strands in the 

 stalk. (From Lang, after Seeliger.) 



