JELLY-FISH, ETC. 



495 



hollow, and somewhat flattened, recalling the shape of a locket. This is the 

 so-called planula, which for a time swims about, then attaches itself firmly by 

 the end of its body and becomes pear-shaped, the stalk of the pear being 

 represented by the attached end ; a horny envelope is then secreted over the whole 

 surface, the mouth breaks through the free end of the central cavity, four tentacles 

 appear, and we have a four-armed polyp or scyphistoma. The tentacles increase 

 in number, and the scyphistoma can produce at its base a number of young polyps 

 which again can multiply by division. At a certain period, this method of 

 multiplication by budding of the polyps from the base ceases, and each scyphi- 

 stoma divides up in quite a different fashion. The polyp becomes horizontally 

 constricted in several places, until it appears like a number of cups placed one 

 inside the other ; this is called a strobila (pine-cone). When ready, the top cup 

 breaks away, turns over, and swims as a young 

 form of medusa, called an ephyra y which gradu- 

 ally acquires the shape of the perfect discomedusa. 

 We thus have here an alternation of generations 

 in which a sexual medusa-generation is succeeded 

 by an asexually- reproducing polyp -generation, 

 this again being followed by another medusa- 

 generation. 



In relation to these, and constituting a kind 

 of transition form connecting the Discomedusas 

 and the polyps, are the Calycozoa, or cup-shaped 

 medusae, which either swim about freely or are 

 attached by their apices, where the firm gelatinous 

 disc attains its greatest thickness. At the margin 

 of the disc, these forms carry eight to sixteen 

 arm -like processes. In the attached forms 

 (Lucernaria) the ends of these processes are 

 provided with short tentacles, occasionally broad- 

 ened into discs and used for attachment, and 

 also with stinging-capsules. The Calycozoa may 

 leave their place of attachment and swim about 

 for a time, with a rotatory motion, and then again 

 settle down. Lucernaria has been found as deep 

 as three thousand three hundred feet, but appears 

 to prefer to settle in shallower water. The 

 nearest relations of Lucernaria are the Tesseridce. These creatures are small and 

 swim about freely, having an elegant long bell-like shape. The edge of the disc is 

 drawn out into alternately longer and shorter arms, eight to sixteen in number. 



Tessera (20 times nat. size). 



THE SEA- ANEMONES ANR CORALS, Class Anthozoa. 



We turn from the free-swimming Scyphomedusae to the permanently fixed 

 polyp forms, namely, the sea-anemones and corals ; the latter of which leave 

 behind them monuments compared with which the pyramids sink into insignifi- 



