526 



COMPARATIVE ANATOMY 



CHAP. 



vesicles, which continue to grow backward in proportion as the posterior part of the 

 archenteron, the larval intestine, grows anteriorly (Fig. 425). 



Fourth day. The whole hydro -enterocoel becomes constricted off from the 

 larval intestine, and is now found as a large vesicle occupying the anterior part of 

 the larval body, and continued posteriorly in the two long hydro-enterocoel vesicles, 

 the one on the left being longer than the one on the right (Fig. 426). 



An imagination of the ectoderm, somewhat anteriorly to the middle of the 

 ventral side, represents the rudiment of the larval mouth and oesophagus, and. 

 towards the end of the fourth day, breaks through into the larval intestine. 



Anteriorly a cushion-like thickening of the body appears encircling a depression. 



ant 



!'!<;. t-Js. Larva of As- 

 terina gibbosa four days 

 old, just hatfheel, from 

 the ventral side (after 

 Ludwig). 1, Larval organ ; 

 2, blastopore. Here, and 

 in the following figures, 

 unt = anterior ; post = pos- 

 terior. 



FN;. 42;. Asterina 

 gibbosa, larva six days 

 old, from tlm left side 

 (after Ludwig). r, Ven- 

 tral side ; </, dorsal 

 side; 1, larval organ. 



FIG. 427. Asterina gibbosa. 

 larva five days old, horizontal 

 longitudinal section seen from tin- 

 ventral side. First rudiment of the 

 hydrocoel outgrowth (7) on the left 

 of the liydro-enteroccel vesicle (3). 

 The two enterocoel vesicles ha\>> 

 opened into one another posteriorly 

 at 8. 



This circular cushion, the rudiment of the larval organ, slants from above anteriorly 

 to below posteriorly (Figs. 428-430). 



At the end of the fourth day the embryo leaves the egg-envelope, and swims 

 about freely by means of the cilia covering its entire surface. 



Fifth day. The two hydro-enterocoel vesicles grow round the larval intestine 

 above and below. Where they meet below, somewhat to the left of the median line, 

 they form a ventral mesentery, which, however, rapidly disappears, the two vesicles 

 opening into one another at this point. Above the intestine a dorsal mesentery, 

 lying to the right of the median line, arises in a similar manner, and persists. 



The left hydro-enteroccel vesicle bulges out laterally somewhat behind its middle 

 point. This bulging is the rudiment of the hydrocoel (Fig. 427). 



At this stage, therefore, the hydro-enteroccel consists of the following sections in 

 widely open communication with one another. 



1. Anterior unpaired enterocoel (6 in Figs. 426, 427), lying in the larval organ. 



2. Right enterocoel vesicle (2 in the figs.), anteriorly in wide open communica- 

 tion with No. 1, and ventrally in open communication with 



