ECHINODERMA TA— ONTOGENY 



529 



The larval organ attains its greatest development on the eighth or ninth day, it 

 iliminislies in sizi' later, and finally is altogether resorbed, without giving origin to 

 any organ of the young Asteroid. Its wall consists of three layers : (1) an outer 

 ciliated larval epithelium, (2) the inner epithelium of the unpaired section of the 

 enterocoel which tills up the whole cavity of the larval organ, and (3) between these 

 two layers, one of mesenchyme cells differentiated into muscle fibres. The larva uses 

 the organ for locomotion and for temporary attachment. 



Soon after the ambulacral (oral) rudiments of the arms have appeared on the left 

 side in the form of the five protuberances (1-5, Fig. 437) mentioned above, five 

 mesenchyme thickenings form, which also bulge out the ectoderm, and represent 

 the antiambulacral (apical, dorsal) arm rudiments (I-Y). Three of these are 



Fig. 435.— Asterina gibbosa, larva six 

 days old, .seen from the left (after Ludwig). 

 1-V, The five primary bulgings of the 

 hydrocoel (7) ; 3, the left enteroccel, opening 

 outward through the hydropore or dorsal 

 pore (11) on the dorsal side ; 5, intestine ; 

 0, anterior euterocrel, enterocoel of the 

 larval organ ; 9, larval mouth ; 10, mesen- 

 tery ; 11, dorsal pore ; 12, ectoderm of the 

 larval organ. 



^\^^^ ik 



Fig. 436.— Asterina gibbosa, larva eight 

 days old, seen from tlie dorsal and somewhat 

 from the left side, optical longitudinal section 

 (after Ludwig). II, V, second and fifth primary 

 bulgings of tlie hydrocoel ; 3, left enteroccel ; 

 5, intestine ; 7, hydrocffil ; 11, dorsal pore ; 

 13, oesophagus of the Asteroid ; 14, rudiment of 

 the stone canal. 



found on the left ventral side, and two somewhat to the left of the median line, on 

 the dorsal side of the larva. The five stand in a curved row, the curve opening 

 anteriorly, the plane in which they lie making an angle with that of the ambulacral 

 arm rudiments. 



These two sets of arm rudiments then shift towards one another, until their 

 planes are nearly parallel. 



Appearance of the skeletal plates. — As early as the time when the hydroccel 

 bulgings begin to become trilobate, a small calcareous body appears in the mesen- 

 chyme on each side of each primary bulging, on the proximal side of the lateral 

 secondary bulgings {i.e. of the rudiments of the first tube-feet). These are the 

 rudiments of the first five pairs of ambulacral plates. When a second pair of lateral 

 lobes appear distally to the first pair on each hydroccel bulging, a second pair of 

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