AHTUL'R HESS 



is practically absent and the two muscle layers are essentially in 

 contact. The ordinary slow moxement which Hydra performs could 

 well be subserved bv muscle to muscle transmission. 



REGIONAL HISTOLOGY 



llic Jiypvstoinc (Figs. 26, 27). The hypostome has a relatively 

 low-lying ectoderm (Fig. 26). The endoderm is extremely well- 

 developed ( Fig. 27 ) . The very large cells and dense accumulations 

 of gland cells sometimes practically obliterate the lumen. 



The peduncle. The endoderm of the peduncle is reduced in 

 extent and gland cells are absent. The digestixe cells consist of 

 very vacuolated thin strands of cytoplasm. The ectoderm is similar 

 to that already described. The epithelio-muscular cells of this region 

 are characterized b\- ha\"ing granules near their surface (see 

 Fig. 30). 



T]u' based disk (Fig. 29). The endoderm of this area is like 

 that of the peduncle. The ectoderm is characterized b\- the presence 

 of a type of gland cell which consists mostl)- of double membranes 

 and has large granules, similar to those seen in the epithelio-mus- 

 cular cells of the peduncle, l)ut much larger. These granules of the 

 gland cells of the ectoderm in this area are apparently the substance 

 produced to cement Hydra to the substratum. The ectodermal 

 cells of the pedal disk have small extensions of cytoplasm or villi 

 on their surface. No granular and cuticular material is present on 

 the surface of Hydra at this level. 



Hie tentacle (Figs. 22, 24, 28). The tentacle arises at the level 

 of the Inpostome. Sections through this region reveal a gradual 

 change of the cells with the tentacle compared to the hypostome 

 having a reduced endoderm and ectoderm, reduced number of 

 gland cells, increased vacuolation of digestive cells, increased num- 

 ber of cnidocytes, and perhaps better de\ elopment of the muscle 

 filaments in the epithelio-muscular cells. The endoderm-mesoglea 

 interface at this level exhibits a characteristic scalloped appearance 

 (Fig. 28). 



The endoderm of the tentacle is severely reduced (Fig. 22). It 

 consists of \'ery thin wisps of cytoplasm of digestive cells enclos- 



