140 C. E. Beecher — Development of the Urachiopoda. 



or arms ; (c) the usual dehiscence of the four bundles of setae ; 

 (d) the obsolescence of the eyes; (e) the definition of the 

 oesophagus and stomach ; and (j) the agreement of the mus- 

 cular system with that in adult forms. These features, with 

 the pedicle which appeared in a preceding stage, represent the 

 brachiopod phylum, and are properly referred to the phylem- 

 bryonic period of Jackson. Although the molluscan stage 

 called the prodissoconch in pelecypods, the 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 brachio- 

 pod, yet there is no homology of distinctive organs. 



15 



16 



Cistella neapolitana Scacchi. 



Figure 15. — Phylembryo., Brachiopod ; showing shell (protegulum). beginning 

 of tentacles of lophophore (I), obsolescence of eye spots, and formation of 

 oesophagus, t. hinge teeth ; vp. ventral pedicle muscles 



Figure 16. — Nepionic brachiopod; showing distinct tentacles of lophophore, 

 mouth and stomach, and transformation of muscles from typembryo, figure 

 10; ad, adductors; di, divaricators: vp, ventr"al pedicle muscles. (15, 16, 

 after Koralevski.) 



The mantle of mollusks is first formed on the posterior 

 dorsal side, and is in the shape of a disc, 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 side 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 



