6o ZOOLOGY 



The generative glands are on the sub-umbrella surface of 

 the radial canals. Zimnocodium, the only freshwater medusa 

 with whose anatomy we are acquainted, is placed by Lankester 

 in this order. It forms an exception to the rule of the absence 

 of a hydroid form. A small hydranth has been described 

 as budding off the medusae, which are found at intervals in 

 tanks of the Victoria regia in the Botanical Society's Garden, 

 Eegent's Park. 



Order 6. Narcomedusae. 



Chaeacteeistics. — The Narcomedusae always have their ten- 

 taculocysts free. The tentacles arise some way up the aboral 

 face of the umbrella, not from its edge. Their bases are con- 

 nected with the edge of the umbrella by stiffening rods of tissue 

 termed Peronia. These may also he found in the Tracho- 

 medusae. 

 The generative organs arise from the sub-umbrella surface 



of the stomach, not on the radial canals. 



Class B. SCYPHOMEDUSAE. 



Chaeaoteristics. — The hydroid stage of this sub -class is a 

 polype, which may give rise to the medusa, not by budding 

 but by transverse fission. It is termed the Scyphistoma. The 

 mouth of the Scyphistoma is squarish, and the coelenteron is 

 sub-divided by four ridges, termed taeniolae, which project 

 from the sides into its lumen. 



The 'medusa has also a square mouth, whose angles may be 



produced into four oral processes. The edge of the umbrella 



is lobed. The sense organs are modified tentacles, into the 



base of which the coelenteron is produced. No continuous 



nerve ring exists, but scattered nerve centres are found round 



the margin, and no true velum is present. Chaeybdaea 



forms, however, an exception to this statement. The generative 



cells are endodermic in origin, and the medusae are unisexual. 



In the Scyphomedusae a primary series of four tentacles 



corresponding to the four angles of the mouth are the first to 



be formed. This affords a convenient method for mapping 



out the regions of the body. The radii on which they are 



