EMBRYOLOGY OF ECHINODERMS. 135 



The larva shown in the figures is known as a Bipinnaria. 

 A Brachiolaria is a larva of the same type, but with a 

 system of fleshy, unciliated arms, known as brachiolar 

 arms, at the anterior end of the body, between the loops 

 3 and 4. 



b. The water-system of echinoderm larvae. 



Before the mouth (Fig. 62, m) joins the stomach (o), 

 two little pouches, the water-tubes, or peritonseal vesicles, 

 are constricted off from the stomach, at the point where 

 the oesophagus is to unite with it. After the ossophagus 

 joins the stomach one of these lies on each side of it as in 

 Fig. 65, w). They then lengthen as shown in Fig. 67, w, 

 and the tip of the one which is on the left in a ventral 

 view unites to the integument of the dorsal surface of the 

 body, and forms an external opening there. 



The two water-tubes now lengthen, as shown at w w' in 

 Figs. 78 and 79, and run backwards onto the sides of the 

 stomach, where they form a pair of flattened pouches. 

 They also run forward, and bending towards each other 

 in front of the mouth, unite to form a single large pouch 

 (Figs. 78 and 79, ww'). 



c. The formation of the echinoderm in the body of the 

 larva. 



The flattened portion of that water-tube which lies on 

 the left of the stomach (Fig. 78, am), now becomes 

 folded out to form five lobes (Fig. 80, am) ; Fig. 81, 

 am 1 , am 1 , am 3 , am 4 , am 5 ), which are to become the 

 water-tubes of the five rays of the starfish. These five 

 lobes are arranged in a rosette, with the one which is to 

 belong to the anterior ray of the starfish (am 3 ) pointing 

 towards the posterior end, and those which are to belong 

 to the two rays of the bivium (am 1 , am 6 ), slightly sepa- 

 rated from each other. 



