582 



BKACHIOPODA. 



is small and the segmentation is complete, the gastmla is formed 

 usually by invagination, and the blastopore closes at the anterior 

 end of the ventral surface. The archenteron gives off two lateral 

 diverticula, which are gradually constricted off from it as the coelomic 

 sacs (Fig. 464, a, b). 



The coelomic sacs extend posteriorly behind the enteron, but the 

 last remnant of the communication between the two is at the front 

 end of the body. The embryo now becomes constricted by an 

 annular furrow into two parts, the anterior of which soon divides 

 in a similar manner, so that three segments are formed (Fig. 464, b). 

 It does not appear, however, that the coelomic sacs participate in 

 this segmentation. The enteron is confined to the two anterior 

 segments, and does not extend into the caudal segment. 



K 



FIG. 465. Three diagrams illustrating the hypothetical metamorphosis of a Brachiopod in 

 relation to that of Actinotrocha (after Korschelt and Heider). A, free-swimming larva show- 

 ing the mouth-opening and commencing tentacles of the lophophore, neither of which are 

 present at this stage. B, a transitional stage showing reduction of supposed preoral lobe. C, 

 young Brachiopod after the turning forward of the mantle lobes. The preoral lobe (cephalic 

 umbrella) has shrunk to the epistome. d dorsal mantle lobe ; ep epistome ; k preoral lobe 

 (cephalic umbrella) with its ring of cilia (trochosphere ring) ; m mouth ; st stalk ; t rudi- 

 mentary tentacles ; v ventral mantle lobe. 



On the first segment (head) there is developed an umbrella-like 

 disc, the edge of which becomes ciliated, and four eye spots. On the 

 second segment (thorax) there is an annular fold (Fig. 464, c), which 

 soon grows out into a dorsal and ventral lobe the mantle-lobes. On 

 the ventral part of this are developed four bundles of provisional 

 setae. 



The mantle-lobes project back over the caudal region (Fig. 464, d). 

 The embryo now leaves the brood-pouch and enters upon its free- 

 swimming life. 



The larval stage is of short duration, and during it the enteron 



