468 



INVERTEBEATA 



CHAP. 



the coelomic vesicles develop muscular fibrils at their bases, which 

 are for the most part disposed circularly, but some of which pursue a 

 longitudinal course. Now the brachiolarian arms differ from the 

 other larval arms in possessing hollow outgrowths from the coelom 

 within them, and it appears certain that the longitudinal muscular 

 fibres accompanying these outgrowths can cause a retraction of the 

 central portions of the tips of the brachiolarian arms, and thus enable 

 them to act as suckers. 



As we have noted above, the right and left coelomic sacs fuse with 

 one another in the prae-oral lobe, and the left becomes almost divided 



bra. m.d.a. 



FIG. 360. Lateral views of an advanced Bipinnaria of Asterias vulgaris, in which the 

 brachiolarian arms are just appearing. (After Goto.) 



A, outline view to show the segmentation of the coelom. B, more detailed sketch. Letters as in 

 Fig. 358. In addition, a.c, anterior coelom ; fer.a, rudiment of anterior median brachiolarian arm ; liy, 

 hydrocoele ; 1-5, the five lobes of the hydrocoele ; l.p.c, left posterior coelom. 



into two by a constriction which appears just behind the hydrocoele. 

 Somewhat later a similar constriction appears in the right coelomic 

 sac, and by these two constrictions two posterior regions, the left 

 posterior coelom and the right posterior coelom, become marked 

 off from the left and right coelomic sacs. The left posterior coelom 

 begins to extend beneath the gut over to the right side ; this extension 

 is known as the right ventral horn of the left posterior coelom, and 

 its formation causes the whole sac to take on the form of a U (Fig. 

 361). It fuses with the right coelomic sac in front of the constric- 

 tion,, separating it into anterior and posterior portions in a word, it 

 fuses with the right anterior coelom. Soon, left and right anterior 

 coelomic sacs, already fused in front, become completely merged in 

 one another so as to form a single anterior coelom. The right 



