XI 



POLYZOA 



397 



eventually cut off from that of the parent zooecium by a mesodermic 

 septum, but before this happens the first rudiment of the new 

 polypide appears in the new zooecium as an ectodermic thickening, 

 which later becomes an ectodermic pouch open to the exterior, 

 except in so far as it is roofed over by the common cuticle or 



Fig. 317. — Stages in the developmeut of tlie bud of Bugula aidcularia. (After Seeliger.) 



A, the early rudiment of the polypide in the form of an ectodermic invagination. B, a later stage : 

 the rudiment of the polypide is almost shut off from the exterior, and has increased in depth, and its 

 mesodermic covering has become continuous. C, D, two sections through an older polypide in which 

 the constriction of the rudiment into atrium and gut has begun. C, is through the opening which 

 remains as the anus. E, section through a still older polypide in which the tentacles have appeared 

 as ridges in the atrial wall. The section goes through the opening which remains as mouth, at, 

 rudiment of atrium ; poLmes, mesoderm ; pol, polypide ; oes, oesophagus ; ten, tentacles. 



ectocyst. This pouch deepens and its mouth closes, and the resulting 

 sac becomes divided by a constriction into an upper region, which 

 is the rudiment of the future atrium or tentacle sheath, with its 

 contained tentacles, and a lower region, which is the rudiment of the 

 entire gut of the new person. 



The sac becomes clothed externally by mesoderm cells ; these 



