CHAMBERS'S INFORMATION FOR THE PEOPLE. 



grows a single polypite, which by budding grows 

 into a form exactly resembling the branched tree- 

 like zoophyte, which gave origin to the Medusa- 

 like bud. This is an example of what Steenstrup 

 called ' alternation of generations,' although it is 



a, Hydromedusarium of Bougainvillia suptrciliaris ; 

 b, Young Bougainvillia, or Medusoid bud, liberated 

 from the Hydromedusarium. 



not so in reality, there being only one truly gener- 

 ative act namely, when the ova and spermatozoa 

 of the Medusa-like bud or zooid unite to produce 

 the disc-shaped ciliated body. 



ORDER 4. The Siphonophora, represented by 

 Physalia (Portuguese man-of-war), Vellella, and 

 by a multitude of wonderfully beautiful oceanic- 

 surface forms, such as Agalma and Dip/tyes, which 

 frequently swarm in the warmer seas, many of 

 them forming branching feathery organisms several 

 yards in length, and consisting of an infinite num- 

 ber of glassy swimming-belts, feeding polypites, 

 reproductive polypites, and filaments covered with 

 thread-cells, many of their parts relieved and 

 brightened by coloured spots, crimson, blue, or 

 purple. Some have a float or inflated bag, which 

 enables them to float on the surface of the ocean. 

 Depending from the under-surface of the animal 

 are a number of thread-like appendages, and a 

 soft flexible stem, to the sides of which are attached 

 numerous buds or polypites. These also repro- 

 duce themselves by Medusa-like buds, like the 

 Sertularia, 



ORDER 5. The true Medusidce have a 

 swimming-bell, like an umbrella, composed of a 

 jelly-like material. A single polypite is suspended 

 from the centre of the concavity of the bell ; 

 and, running inwards from its margin is a veil, 

 perforated in the centre by an aperture, through 

 which water is admitted into the interior of the 

 bell, and is again expelled by its rhythmical con- 

 tractions, so that the animal moves in a direction 

 opposite to that in which the water is expelled. 

 Four canals, radiating from the centre, run in its 

 walls, and are joined at the circumference by a 

 circular canal Suspended from the margin of the 

 bell is a number of tentacles, and on it are 

 several coloured spots, which are supposed to 

 represent rudimentary eyes. These Medusae pro- 

 duce animals exactly resembling themselves. 



ORDER 6. Discophora, In the interior of 



136 



old shells may be found an animal closely resem- 

 bling Hydra, but sexless. It is called Hydra 

 tuba. At certain seasons, a number of transverse 

 markings may be seen upon a cone-like append- 

 age growing from the free end of the hydra, which 

 gradually deepen into annular constrictions, so 

 that it looks like a pile of saucers. Each segment 

 is thus a transverse segment of the cone-like body 

 projecting from the free end of the hydra tuba. 

 Each develops tentacles, and becoming detached, 

 grows into a hooded-eyed Medusa. This, when 

 perfectly developed, consists of an umbrella or 

 swimming-bell, with one or more polypites sus- 

 pended from its concavity. It has eight radiating 

 canals, whose branches anastomose with each 

 other ; but it has no veil. It has a number of 

 tentacles suspended from the margin of the 

 umbrella, and upon it are coloured eye-spots, and 

 small cavities containing a fluid with crystals of 

 carbonate of lime. These last are supposed to 

 represent organs of hearing. Both of these are 

 covered over by a fold of membrane like a 

 hood, and hence 

 the name 'hooded- 

 eyed ' Medusae. 

 They are often of 

 enormous size, and 

 are commonly 

 called jelly-fish or 

 sea-blubbers. They 

 are represented by 

 such forms a.sRhtz- 

 ostoma and Pela- 

 gia. These pro- 

 duce ova, which 

 again, in turn, give 

 rise to hydra tuba, 

 affording another 

 example of the so- 

 called 'alternation 

 of generations.' The phenomenon of phosphor- 

 escence of the sea is largely due to the Medusidas, 

 and some of the Discophora, which, when irritated, 

 emit sparks of light, or glow like globes of fire. 



CLASS II. ACTINOZOA. 



The digestive sac is distinct from, and sus- 

 pended in, the body cavity, communicating freely 

 with it by an opening below. The space inter- 

 vening between the stomach and the body-wall is 

 called the ' peri-visceral space,' and is divided into 

 a series of chambers by vertical partitions, to the 

 faces of which the reproductive organs are at- 

 tached. 



Actinia is the type (aktis, a ray). The body 

 is short, and of fleshy consistence, attached at 

 one end to a rock, and having at the other 

 the mouth, surrounded by several rows of 

 tentacles, which, from their arrangement and 

 colour, like that of a full-blown many-petalled 

 flower, have obtained for this animal its com- 

 mon name of sea-anemone. The mouth leads 

 into a stomach, opening at its lower end 

 into the peri-visceral cavity, which is divided 

 by vertical partitions, having the reproductive 

 organs attached to them. They feed upon shell- 

 fish and other marine animals, which they draw 

 into the mouth by the tentacles, disgorging shortly 

 afterwards the shells and other indigestible parts. 

 They are very sensitive to light, and expand or 



Rhizostoma. 



