xii BKACHIOPODA 411 



the part of the larva behind the posterior mantle-groove is the " foot 

 segment," the middle region being known as the " mantle segment." 

 The posterior groove is deepest on the dorsal side, the anterior groove 

 on the ventral one (Fig. 325). The mantle-fold grows rapidly back- 

 wards until it almost completely covers the foot segment. At 

 four places on the edge of the mantle, two near the mid-dorsal line 

 and two more laterally situated, invaginations are formed, and the 

 cells at the bottom of these sacs give rise to long chaetae. These 

 chaetae were not observed by Conklin, but the sacs were visible in 

 his sections, the chaetae had dropped out. 



Whilst these changes have been taking place the coelom has been 

 undergoing development. When we last referred to it, it consisted of 

 right and left sacs which communicated with one another in front 

 and behind. These communications, however, become closed by the 

 proliferation of cells into the 

 cavity of the coelom. The 

 coelomic cavities become 

 narrow, almost vestigial, in 

 the region of the foot, but in 

 the head region they expand 

 and take on a trefoil form. 

 In the mantle region the 

 coelom sends out dorsal and 

 ventral extensions into the 

 mantle-folds. This section of 

 the coelom alone remains wide 

 and spacious; the cavities of 

 the head and foot coelom 

 almost disappear in conse- 



,t .1 i . ,. Letters as in preceding figure. 



quence ot the thickening ot 



their lateral walls. When viewed from the side, the coelom is seen 

 to be regularly divided into head-, mantle, and foot lobes, and 

 there is a specially narrow part in the middle of the foot behind 

 which it widens out again. This last dilatation is regarded by 

 Shipley (1883) as forming a fourth segment (Fig. 325). 



The gut is a flask-shaped sac, the broad end of which is anterior 

 and the narrow end posterior. In the latest stage which Conklin 

 examined, a ventral outgrowth of endoderm cells is formed, extending 

 towards the pit already referred to, which marks the site of the last 

 trace of the blastopore. There is no doubt that this is the rudiment 

 of the endodermal part of the oesophagus. 



Just in front of the dorsal apex of the larva the ectoderm is 

 slightly invaginated. The invaginated cells are very long and slender 

 and carry a specially long tuft of cilia (Figs. 326 and 327). At 

 their bases small rounded ganglion cells are cut off, which, how- 

 ever, remain in close contact with the ectoderm. The plate so formed 

 is evidently homologous with the apical plate of the Trochophore 

 larva, and the cells at its base are the rudiment of a supra- 



