THE HYDROMEDUSAE 



neck, are formed and lie inside the vesicle, and are everted only by 

 pressure upon its walls. Two kinds of nematocyst, a larger and a 

 smaller, are generally present, and exhibit some differences of 

 detail. Gland cells and pigment cells are not uncommon. Multi- 

 polar ganglion cells, lying beneath the surface of the ectodermal 

 epithelium, have been detected in numerous species. The smaller 

 interstitial cells, of irregular form, which fill the interspaces between 



N 



IIP. ?. 



FIG. 9. 



FIG. 10. 



8. Types of Hydrouiedusan cells, after von Lendenfeld and Schulze. 1, epithelio-nmscular 

 cell, with palpocil and contractile processes ; 2, 3, muscular cells showing the transition from 

 the epithelioid to the fibrous condition ; 4, sense cell with palpocil, connected by nerve fibre 

 with ganglion cell ; 5, supporting cell with palpocil ; 6, cnidoblast, with three cnidocils, en- 

 closing a nematocyst, and connected by nerve fibre with ganglion cell ; 7, endodenn cell with 

 ciliuin ; the protoplasm is vacuolated and contains (?) food particles ; 8, amoeboid cell from 

 mesogloea. 



9. Cnidoblast with cnidocil and nematocyst ; the thread and barbs of the latter have 

 been everted. (After Schulze.) 



10. Vacuolated endodenn cells of " cartilaginous " consistence from the axis of the tentacle 

 of Cunina. (From Gegenbatir's Elements of Comparative Anatomy.) 



the others, are apparently differentiated as required into the more 

 specialised cell forms already mentioned. 



The endoderm is also generally composed of a single layer of 

 cells, and is ciliated ; there is generally one cilium on each cell, 

 which is capable of withdrawal. The larger cells of the endodermal 

 epithelium are essentially digestive cells, but are in many cases also 

 provided with short contractile fibres which lie on the mesogloea 



