250 



STUDIES IN EVOLUTION 



cephalic border, and the organism becomes a free-swimming 

 larva (figures 91-93). 



Larval Stages. 

 The Typemhryo is the larval stage at which some distinc- 

 tive features make their appearance, but before the special 

 characters of the class are to be found (figure 94). It is 

 analogous to the molluscan embryo in which a shell gland 

 and plate-like initial shell are developed. There is, however, 

 no homology of parts or organs between the tj-pembryonic 

 mollusk and brachiopod. 



-^^ 



Cistella neapolitana Scacchi. 



Figure 93. — Neoembryo ; completed cephalula stage. 



Figure 94. — Typembryo; transformed larva resulting from folding up- 

 ward of mantle lobes over cephalic segment, ad, muscles from bundles of setse 

 to sides of body cavity ; di, muscles from dorsal to ventral sides of body ; rp, 

 muscles from ventral side of body to caudal segment or pedicle. (Figures 93 

 and 94, after Kovalevski.) 



In Cistella and Terehratulina the development of the typ- 

 embryo has been observed, and consists of the folding 

 upward of the lobes which have been developed from the 

 thoracic segment to form the mantle, so that they gradually 



