528 



COMPARATIVE ANATOMY 



CHAP. 



The two lateral mesoderm vesicles lengthen and fuse in front of and above the 

 mouth, and, further, surround the intestine. On the left vesicle (hydro-en terocoel 

 vesicle) a transverse constriction a^jpears, which finally divides it into two vesicles, 

 an anterior, which at its posterior end opens outward through the stone canal and 

 water pore, and a posterior (Fig. 433). 



Further development of the hydroccel in Asterina gibbosa. — After the seventh 

 day the five outgrowths of the hydroccel (Figs. 437-440) become trilobate, and 

 later have five lobes. The unpaired terminal lobe of each outgrowth is the rudiment 

 of the terminal tentacle, the paired lobes are the rudiments of the first two pairs 



Fig. 432. —Larva or 

 Asterias vulgaris, about 

 four days old, from the 

 dorsal side (after Field). 

 1, Circuinoral ciliated 

 band ; 2, u.outh ; 3, right 

 and left hydro-enteroc(j_'l 

 vesicles, with their hydro- 

 pores (4) ; 5, cesopliagus; 6, 

 niesenchyniatous muscle 

 fibres; 7, stomach intes- 

 tine ; 8, anus. The month 

 and the anus lie on the 

 side turned away from 

 the reader. 



Fig. 433.— Dorsal aspect of a 

 Bipinnaria larva to illustrate tlie 

 development of the liydro-enteroccBl 

 (after Bury). 1, Larval cesopliagus ; 

 2, left anterior enteroccel ; 3, hydro- 

 pore ; 4, rudiment of the hydroccel ; 

 5, stomach intestine; 6, terminals; 

 7, left posterior enterocoel vesicle ; 

 S, dorsal mesentery ; 9, riyht posterior 

 enteroccel vesicle ; 10, madreporite ; 



11, blood vesicle, pulsating vesicle; 



12, right anterior enteroccel. 



2)ost 



Fig. 434.— Asterina gib- 

 bosa, larva six days old, 



horizontal longitudinal sec- 

 tion from the ventral side 

 (after Ludwlg). The hydro- 

 ccel (7) has become con- 

 stricted posteriorly from the 

 left enterocoel. An out- 

 growth of the intestine (8) is 

 the first indication of the 

 future o-sophagus of the 

 Asteroid. 



of tube-feet. Each new pair of feet arises between the terminal tentacle and the 

 foot last formed. 



The five outgrowths of the hydroccel become outwardly visible, bulging out the 

 body. On the left side of the seven-days-old larva there are thus visible five flat 

 protuberances arranged in a convex arch directed upward and backward ; these 

 protuberances become more marked on the eighth day, and are then divided either 

 into three or five lobes each (Figs. 438-440). These are the first indications of the 

 young Asteroid, the rudiments of its ambulacra! arms. 



The rudiment of the definitive oesophagus appears in the form of a bulging of 

 the left side of the archenteron, that facing the hydroccel. This arises in the region 

 which corresponds with the anterior part of the gastrula intestine (Fig. 434, 8), and 

 has nothing to do with the larval oesophagus. This latter degenerates on the 

 eighth or ninth day, and the larval anus also disappears. 



