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 difterences of 

 detail. Gland cells and pigment cells are not uncommon. ]\Iulti- 

 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 



; 7 



I ir. f . 



Fig. 9. Fio. 10. 



8. — Types of Hydromedusaii cells, after von Lendeiifeld and ScliuUe. l,epithelio-niiiscular 

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

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

 with <,'ani,'lion cell ; 5, supporting cell with palpocil ; (J, cnidoblast, with three cnidocils, en- 

 closing a nematocyst, and connected by ner\'e fibre with ganglion cell ; ~, endoderm cell with 

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

 laesogloea. 



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

 been evei-ted. (After Schulze.) 



10. — Vacuolated endodi-rm cells of "cartilaginous" consistence from the axis of the tentacle 

 of Cunina. (From GegenUiur's El'vienh ofComjHiwtive Anatomy.) 



the others, are apparently ditterentiated 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 



