16 JOURNAL OF MARINE ZOOLOGY AND MICROSCOPY. 



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 alternate with the perradials. Between each primary and secondary 



radius lies one of eight radiating genital bands. Their position is tech- 

 nically known as adradial. The margin of the bell just beyond the 

 extremity of each genital band is marked by a great cluster of short 

 capitate tentacles. The remaining external organs are eight con- 

 spicuous hollow bodies, each one lying midway between every two 

 groups of tentacles. These are probably vestigial sense organs and 

 have been termed colleto-cystophores. They will be referred to later 

 as.c.-c. Four must be interradial and four perradial. Looking to 

 the internal anatomy, we find that the enteric or body-cavity is 

 divided by four delicate radiating walls or septa into four gastric 

 chambers — perradial in position. The septa— which by the way 

 represent the mesenteries of the Anemones — are interradial. From 

 their inner edges are given off numerous solid tentacle-like filaments 

 — the gastral filaments. 



Those unfamiliar with these animals are reminded that the 

 Hydrozoa are divided into the two great groups, Scyphomedusse and 

 Hydromedusse. The former includes, as principal order, those fleshy 

 medusas or Jelly-fishes — without a velum (membrane all but closing 

 the mouth of the bell), with gastral filaments and with eight sensory 

 organs (tentaculocysts) — known so well under the forms of Aurelia 

 and Pelagia and termed collectively Discomedusas. Normally the 

 life-history of these animals is as follows : — A ciliated embryo (pro- 

 ceeding from the sexual or medusa stage), after a short period of 

 pelagic existence, settles down and becomes attached to some sta- 

 tionary object. A mouth forms at the free end, and sixteen tiny 

 tentacles appear. This form, A- to 1-in. in height, is the scyphistoma 

 stage and is often termed the hydriform phase from its likeness to 

 the fresh-water polyp, Hydra. Soon transverse constrictions — stro- 

 bilation — cut the body into a rouleau of discs, appearing thus as a 

 tiny pile of sculptured plates. Each disc has eight arms, with a tiny 

 tentaculocyst at tip. This is the strobila condition. The discs 

 break off and begin a free swimming life, known at this period as 

 ephyrce, rapidly take on the form of ordinary Jelly-fishes, develop 

 sexual organs, and send out swarms of ciliated embryos to go through 

 the same strange cycle of life. 



The Lucernarians form another order of the Scyphomedusse. 

 Contrasting their life-history with that of the large fleshy medusas 

 just related, we find that the Lucernarians produce in the genital 

 bands both ova and sperm masses. These give rise to ciliated 

 embryos which settle down quickly and develop without metamor- 

 phosis into the adult bell-shaped form. 



Until recently HaZiclystus and the related forms were considered 

 as being more or less unchanged descendants of the ancestral form of 



