54 CONKLIN — EMBRYOLOGY OF A BRACHIOPOD. [April 4, 



that the posterior mantle furrow is deeper and is farther forward on 

 the dorsal than on the ventral side (Fig. 31, Fp). 



When the mantle has reached the limit of its backward growth it 

 entirely covers the peduncle, the extremity of which nearly fills the 

 circular opening of the mantle chamber (Figs. 34 and 36). This 

 is the oldest stage which I have had opportunity to study. Morse 

 C 11) has observed in detail the transformation of this larva into the 

 adult. He figures a great many stages showing the manner in 

 which the mantle is turned forward over the cephalic region, its 

 free edge being directed forward and its originally internal surface 

 becoming external. This happens after the larva has become fixed 

 by the tip of its peduncle, and it is evident from Morse's figures 

 and descriptions that the stage shown in my Figs. 34-36 is one of 

 the last stages in the free-swimming life. 



2. The Cephalic Regmi lies in front of the anterior mantle fold 

 and is nearly hemispherical in shape, being however somewhat 

 variable in form (consult Figs. 32-36), which is probably due to 

 the fact that it is extremely contractile, as Morse has observed. At 

 its anterior end and slightly toward the dorsal side is a shallow 

 depression, the apical sense plate, which bears a tuft of long cilia 

 in life (see Morse). The enlarged end of the enteron as well as a 

 portion of the mesoderm and coelom extend into the cephalic 

 region. 



3. The Pedunculai' Region is cylindrical in shape and is con- 

 tracted near its posterior end. This contraction is due to the fact 

 that the coelom and mesoderm terminate abruptly some distance 

 in front of the end of the peduncle (Figs. 34 and 36), and it is 

 certainly not to be taken as constituting a fourth region of the 

 larva, as Shipley ('82) suggests in the case of Cistella. The endo- 

 derm is continued as a solid cord of cells nearly to the end of the 

 peduncle. 



4. The Ectoderm covering the larva is unusually thick, though 

 consisting of but a single layer of cells ; these cells are however 

 extremely long. Their inner ends are clear and free from nuclei 

 and granules, so that on first examination a clear zone seems to 

 separate the outer from the inner layer (Figs. 38 et seq.). Only on 

 the anterior side of the mantle fold does the ectoderm become 

 cuboid or squamous, while over the cephalic and peduncular regions 

 it is particularly thick. 



5. SetcB Sacs. — High columnar cells line the peduncular chamber, 



