500 



PEOF. ALLMAN ON THE STRUCTURE AND 



posed of two concentric cellular layers — the internal derived from 

 the ectoderm, the external from the endoderm of the cystid, while 

 the mesoderm of the cystid takes no part in the formation of the 

 bud. The point where the cystid walls have become invaginated 

 to form the bud corresponds in the completed polypide to the 

 spot at which the tentacular sheath passes into the wall of the 

 cystid ; while the blind end of the sac corresponds to the blind 

 end of the future polypide stomach, that from which at a later 

 period the funiculus proceeds — an organ, however, whose genesis 

 Nitsche has not succeeded in tracing. 



Polds and secondary introversions of this two-layered cellular 

 sac give to the young polypide its definitive form. First, two 

 lateral introversions of the posterior part of the sac grow towards 

 one another, and finally meeting convert this part into a bent 

 tube, each of whose arms opens into the still unchanged anterior 

 part. The bent tube becomes the alimentary canal, and the two 

 openings by which its lumen communicates with that of the ante- 

 rior part of the sac are the oral and anal orifices of the future 

 polypide ; while the anterior unchanged portion of the sac is to 

 become the tentacular sheath. 



The alimentary canal and tentacular sheath thus sketched out 

 consist each of two cellular layers. The inner epithelium of the 

 alimentary canal is derived from the ectoderm of the cystid, while 

 the outer is derived from the endoderm. The two layers of the 

 tentacular sheath have a precisely similar derivation. 



There next occurs, right and left of the oral orifice, a conical 

 introversion of the walls of the alimentary canal. There are thus 

 formed two hollow cones, whose lumen is in each accessible from 

 the body-cavity of the cjstid by a wide opening. These are the 

 foundation of the two arms of the crescentic lophophore. In 

 the further course of the development they become united by a 

 ridge, which runs round the abanal margin of the mouth. This 

 ridge is formed by an infolding of the two layers of the bud, and 

 constitutes the foundation of the abanal or middle portion of the 

 lophophore. The lophophore is thus in its essential features 

 sketched out, and from this the tentacles arise as hollow protru- 

 sions of the lophophore walls. Each tentacle thus represents a 

 short csecal tube which projects free into the cavity of the original 

 cellular sac of the bud, and has its lumen in connexion with the 

 cavity of the cystid, never with that of the polypide. The ten- 

 tacles arise nearly simultaneously from the entire margin of the 



