MORPHOLOGY AND DEVELOPMENT OF EUDENDRIUM. 



33 



Hypostome. One marked peculiarity of the hydranth is the 

 unusual extent to which the hypostome may be expanded or 

 contracted. As a result we have interesting conditions in the 

 muscle processes and the thread cells. A glandular ring fre- 

 quently found near the base of the hydranth is in part like a 

 structure described by Weismann for two species of the genus 

 Eudendrium> and not elsewhere recorded (Fig. 6). 



C 



Fir,. 5. 



The ectoderm cells of the hypostome vary greatly in form 

 during different phases of the contraction or expansion of the 

 hypostome. They appear as pavement cells in the extended 

 organ. If extremely contracted they are not only columnar, 

 but the free ends bulge outward. Their cytoplasm is granular 

 and often vacuolated. The nucleus, like that of entoderm, is 

 large, with a nucleolus and at times a chromatin reticulum. 

 Thread cells occur in a definite ring below the mouth (Fig. 5). 

 Since the tentacles are largely destitute of them, it is apparent 

 that the hypostome has usurped part of the protective function 

 which belongs to the tentacles in most gymnoblastic hydroids. 

 The entoderm of the hypostome is thicker than the ectoderm and 

 consists of more diversified elements. The cells are cubical or 

 columnar, depending upon the degree of extension of the hypo- 

 stome. Greenish-yellow pigment granules occur here and there 



