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, 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 Discomedusae 
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 and Corals,— Class Anthozoa. 
We turn from the free-swimming Scyphomedusse 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- 
