08 ZOOLOGY. 



the disk, though it is produced from a Scyphistoma not 

 more than half an inch in heiglit. 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 tishes, 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. SijjhonopJiora. — 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. 



