DEVELOPMENT OF THE BRACHIOPODA 255 



protoconch in cephalopods and gastropods, and the periconch 

 in scaphopods, represents the completed phylembryo of these 

 groups, as the protegulum represents a like period in the 

 developing brachiopod, yet there is no homology of distinc- 

 tive organs. 



The mantle of mollusks is first formed on the posterior 

 dorsal side, and is in the shape of a disk, which gradually 

 envelops the animal to a greater or less extent, and may 

 become distinctly lobed. As has been shown, this organ in 

 the brachiopods develops simultaneously from the dorsal and 

 ventral sides of the thoracic segment of the cephalula, and is 

 primarily bilobed. 



The initial shell of brachiopods is not produced from a dis- 

 tinct shell gland, as in the Mollusca, but is an integument 

 of the surface of the mantle lobes, and intimately connected 

 with them. The position of the valves is dorsal and ventral. 

 The pedicle has no organic similarity with either a foot or a 

 byssus. 



The mouth of mollusks (and annelids) is formed below the 

 base of the cephalic lobe of the cephalula, and may be the 

 blastopore, while in the brachiopods it is near the anterior 

 pole within the cephalic segment. Notwithstanding these 

 differences, so many parts are functional equivalents that 

 their growth and development may be discussed and inter- 

 preted in the same terms. 



Before passing to later stages of growth which become 

 more and more divergent from a common simple type, some 

 points previously omitted, relating to Thecidium (Lacazella), 

 Lingula (Grlottidia), and Ditcinitca, should be here noted. 

 As Lacazella is a form in which the ventral valve in the 

 neanic and ephebic stages is cemented to foreign objects by 

 calcareous fixation, it bears about the same relation to other 

 brachiopods that Ostrea bears to Avicula, among the pelecy- 

 pods, and a corresponding early absence or modification of 

 many features present in adult individuals should be looked 

 for. From what is known of the geological history of The- 

 cidium, and if the interpretations of its phylogeny by the 



