CCELENTERATA : LUCERNARIDA. 137 



CHAPTER X. 



LUCERNARIDA, GRAPTOLITID^E, AND 

 HYDROCORALLIN&. 



SUB-CLASS IV. LUCERNARIDA (Acalephce, in part). The mem- 

 bers of this sub-class may be defined as Hydrozoa " whose 

 hydrosoma has its base developed into an l umbrella? in the walls 

 of which the reproductive organs are produced" (Greene). 



A large number of forms included in the Lucernarida were 

 described by Edward Forbes under the name of Steganophthal- 

 mate Medusa, being in many external characters closely similar 

 to the Medusidce. These " hidden-eyed " Medusa are familiar 

 to every one as " sea - blubbers " or "sea-jellies," and they 

 occur in great numbers round our coasts during the summer 

 months. The resemblance to the little jelly-fishes is especially 

 strong between the disc or "nectocalyx" of the true Medusidce 

 and the " umbrella" of the Lucernarida, the latter being often 

 a bell - shaped swimming organ, with marginal tentacles, and 

 containing one or more polypites. These analogous struc- 

 tures (figs. 53 and 59) are, however, distinguished as follows : 

 i. The " umbrella" of the Lucernarida is never furnished with 

 a " velum," as is the nectocalyx of the Medusidce. 2. The radi- 

 ating canals in the former are never less than eight in number, 

 and they send off numerous anastomosing branches, which 

 join to form an intricate network ; whereas in the latter they 

 are rarely more than four in number, and though they may 

 subdivide, they do not anastomose. 3. In the place of the 

 separate and unprotected " vesicles " and " ocelli " of the 

 Medusidce, the marginal bodies of the Lucernarida consist of 

 these bodies combined together into single organs, which are 

 termed "lithocysts," and which are protected externally by a 

 sort of hood. 



The Lucernarida admit of being divided into three orders 

 viz., the Lucernariadce, the Pelagidce, and the Rhizostomidce. 



ORDER I. LUCERNARIAD^E. This order includes those Lu- 

 cernarida which have only a single polypite, are fixed by a proxi- 

 mal hydrorhiza, and possess short tentacles on the margin of the 

 umbrella. The reproductive elements " are developed in the primi- 

 tive hydrosoma without the intervention of free zooids " (Greene). 



In Lucernaria (fig. 57), which may be taken as the type of 

 the order, the body is campanulate or cup-shaped, and is 

 attached proximally at its smaller extremity by a hydrorhiza, 



