JELLY-FISHES. 



107 



stem is developed, and the nectostem has little in common with that of Agalma. The 

 nectocalices are of characteristic shape and different from those of any- other siphono- 

 jjhore. Each bell has the shape of a horse's hoof, 

 and has a very shallow cavity and rigid walls. As 

 far as yet known the Hipjjopodise have no diphyi- 

 zooids such as exist in several genera of the Diphys3. 



Obdeb IV.— DISCOIDEiE. 



Among the many interesting forms of Medusae 

 related to, and by most naturalists included in the 

 Siphonophora, are two beautiful genera called Velella 

 and Porpita. These, with a genus JRataria, which is probably the young of one or 

 the other, make uj) a group called the Discoidese. 



Velella has borne the name which designates its most striking peculiarity since the 

 middle of the fifteenth century, on account, perhaps, of a somewhat fanciful likeness 

 to a little sail. It is commonly called in Florida, where it is sometimes very abundant, 

 the " float," and is likewise commonly confounded with the Physalia or Portuguese 

 man-of-war. The body or disk of Velella has an oblong shape, flattened upon its 



JFlG. 101. — Dipliyizooid of Praya. 



Fig. 102. —Velella limbosa. 



Tipper and lower sides. The float is composed of a number of concentric compart- 

 ments in free communication with each other, seven of which open externally in a line 

 extending diametrically across the disk. In the whole diameter there are fourteen 

 such openings, seven in each radius. 



A triangular sail rises on the upper side of the Velella disk and extends diagonally 

 across its surface. It is firmly joined to the upper plate of the float. Over the trian- 

 gular sail as well as the float, there is stretched a thin, blue-colored membrane, which is 

 continued into a variegated soft rim along its border and around the rim of the float. 

 In our most common American FeMa, whicli often reaches a length of four or five 

 inches, the portion of the rim of this membrane around the disk is entire ; in some 

 species, however, it is continued inUo elongated appendages. 



The most important appendages are found on the under side of the Velella disk 



