68 ZOOLOGY. 
the disk, though it is produced from a Scyphistoma not 
more than half an inch in height. Pelagia campanella and 
a few other forms do not undergo this metamorphosis, but 
grow directly from the eggs, not having a Strobila stage. 
Various boarders or commensals—viz., temporary non- 
attached parasites—live in or under the mouth-cavity or be- 
tween the four tentacles of the larger Acalephs. Such is the 
little Amphipod Crustacean, Hyperia, which lives within 
the mouth, while small fishes, such as the butter-fish, swim 
under the umbrella of the larger jelly-fishes, Cyanea, etc., for 
shelter and protection. Besides small animals of various 
classes, the larger jelly-fishes kill by means of their nettling 
organs small cuttle-fishes and true fishes, the animals being 
paralyzed by the pricks of the minute barbed darts. 
Order 3, Siphonophora.—These are so-called compound 
Hydroids, living in free-swimming colonies, consisting of 
polymorphic individuals, or, more properly speaking, zooids 
—that is, organs with a strongly marked individuality, but 
all more or less dependent on each other. A Siphonophore, 
such as Physalia, for example, may be compared to a so- 
called colony of Hydractinia, in which there are nutritive 
and reproductive zooids and medusa-buds. In Physalia 
there are four kinds of zooids—i.e. (1) locomotive, and (2) 
reproductive, with (3) barren medusa-buds (in which the 
proboscis is wanting), which, by their contractions and 
dilatations, impel the free-swimming animal through the 
water ; in addition, there are (4) the feeders, a set of di- 
gestive tubes which nourish the entire colony. There are 
numerous genera and species (one hundred and twenty are 
known), whose structure is more or less complicated and 
difficult to understand without many figures and labored 
descriptions. We will select as a type of the order our 
Physalia Arethusa of Tilesius, or Portuguese man-of-war 
(Fig. 49), which is sometimes borne by the Gulf Stream as 
far north as Sable Island, Nova Scotia. It is excessively 
poisonous to the touch, and in gathering specimens on the 
shores of the Florida reefs we have unwittingly been stung 
by nearly dead, stranded individuals, whose sting burns like 
condensed fire and leaves a severe and lasting smart. 
