CCELENTERATA : HYDROZOA. 99 



parent glass, adorned here and there with the most brilliant 

 colours. They occur in their proper localities and at proper 

 seasons in the most enormous numbers. They are mostly 

 phosphorescent, or capable of giving out light at night, and 

 they appear to be one of the principal sources of the lumi- 

 nosity of the sea. It does not seem, however, that they 

 phosphoresce, unless irritated or excited in some manner. 



CHAPTER XL 

 LUCERNARIDA AND GRAPTOLITID^E. 



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

 bers of this sub-class may be denned as Hydrozoa, " whose hy- 

 drosoma has its base developed into an ' umbrella] in the walls of 

 which the reproductive organs are produced' 1 (Greene.) 



A large number of forms included in the Lucernarida were 

 described by Edward Forbes under the name of Steganophthal- 

 mate Meduscz, being in many external characters closely similar 

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

 to everyone 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 structures 

 (figs. 20 and 24) 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 

 radiating 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 Lticernarida 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 Luccrnariadce, the Pelagidce, and the Rhizostomidce. 



