166 THE MICROSCOPIST. 



It has the form of a polyp, with long slender tentacles. 

 Besides these tentacles with their lasso-cells, it has no 

 special organs except a mouth and tubular stomach. Like 

 the fabled Hydra, if its head be cut off another will grow 

 out, and each fragment will in a short time become a per- 

 fect animal, supplying whatever is wanting, hence its name 

 (Plate XIV, Fig. 124). The Hydra has the power of lo- 

 comotion, bending over and attaching its head until the 

 tail is brought forward, somewhat after the manner of a 

 leech. 



Compound Hydroids may be likened to a Hydra whose 

 buds remain attached and develop other buds until an 

 arborescent structure, called a polypary, is produced. The 

 stem and branches consist of fleshy tubes with two layers, 

 the inner one having nutritive functions, and the outer 

 secreting a hard, calcareous, or horny layer. The indi- 

 viduals of the colony are of two kinds, the polypite or 

 nutritive zooid, resembling the Hydra, and the gonozooid, 

 or sexual zooid, developed at certain seasons in buds of 

 particular shape. 



To mount compound Hydrozoa, or similar structures, 

 place the specimen alive in a cell, and add alcohol drop 

 by drop to the sea-water ; this will cause the animals to 

 protrude and render their tentacles rigid. Then replace 

 the alcohol with Goadby's solution, dilute glycerin, or 

 other preserving fluid. * 



VII. ACALEPHS, or sea-nettles, are of all sizes, from an 

 almost invisible speck to a yard in diameter. They swarm 

 in almost every sea, and are frequently cast upon the 

 beach by the waves. They are transparent, floating free, 

 discoid or spheroid, often shaped like a mushroom or um- 

 brella, and their organs are arranged radiately round an 

 axis occupied by the pedicle or stalk. They are furnished 

 with muscular, digestive, vascular, and nervous systems. 

 They were formerly divided into 



1. Pulvnonigrada, from their movements being eifected 



