520 



COMPARATIVE ANATOMY 



CHAP. 



the blastula stage, and their flagella are less movable. This differentiated part 

 (larval sensory organ ? neural plate ?) can also be recognised in the subsequent 

 stage. 



First Pluteus stage. The gastrula becomes concave on the ventral side ; on the 

 opposite (dorsal) side it becomes convex. The larva is now bilaterally symmetrical. 

 The blastopore at first indicates the posterior end ; then it shifts somewhat on to 

 the ventral side on to a mound-like bulging of the body (the anal area), which lies 

 posteriorly to the ventral depression. The anterior edge of this anal area becomes 

 drawn out into two anteriorly diverging processes, the two posterior ventral arms 



S 



FIG. 416. EcMnocyamus pusillus, gas- 

 trula, forty hours after fertilisation (after 

 Th^el). 1, Blastocoel; 2, frontal thickening 

 of the ectoderm ; 3, mesenchyme cells ; 

 4, formation of these wandering cells at the 

 base of the archenteron ; 5, the first two cal- 

 careous spicules ; 6, archenteron ; 7, primitive 

 mouth, blastopore. 



Fie. 417.- Echinocyamus pusillus, young 

 Pluteus, about forty-eifiM hours after fertil- 

 isation (after The'el), from the ventral side. 

 1, Rudiment of the larval mouth ; 2, the tirsi 

 arms ; 3, rudiment of the hydro-enteroccel 

 at the base of the archenteron ; 4, larval 

 skeleton ; 4i, dorsal branches of the same : 

 5, archenteron ; 6, primitive mouth, blasto- 

 pore. 



(Fig. 417). The circumoral ciliated ring, which is continued on to the arms, 

 becomes raised above the general ciliated covering of the body. 



During the first larval stage the following important internal changes take place. 

 The two first lateral calcareous spicules develop in the mesenchyme and send sup- 

 porting rods into the only two arms present at this stage, the posterior ventral arms. 

 (The first rudiments of these two spicules can be made out even in the gastrula 

 larva, Fig. 416.) 



Formation of the hydro-enterocoel. The anterior blind end of the archenteron 

 has, on each side, a small outgrowth, which lengthens posteriorly. The archenteron 

 becomes constricted immediately behind these outgrowths, which finally become 

 separated from it in the form of a horse-shoe-shaped vesicle, with two limbs directed 

 posteriorly and applied to the archenteron. This hydro-enteroccel vesicle at once 

 divides into its two lateral limbs, forming two lateral hydro-enterocoel vesicles. In 



