HTDROZOA 91 



hydranths (Fig. 152), the stalks on which they grow, the hydrocaulus, 

 and the root-like projections by which the stalk is attached to the sub- 

 stratum, the hydrorhiza. The gastro vascular space (Fig. 150) extends 

 throughout the colony so that all the polyps are in communication with one 

 another. A cuticular layer called the perisarc is secreted by the ectoderm 

 of many species which gives rigidity to the whole colony: in the Hydro- 

 corallinae the perisarc is calcareous and so enormously thickened that the 

 colony has the appearance of coral. The mouth of the hydroid is ter- 

 minal in position and at the summit of an elevation called the hypostome. 



In very many Hydromedusae, the hydroid individuals are polymor- 

 phic (Fig. 162), being specialized to perform different functions. 



The medusoid stage or gonosome is either a free-swimming craspedote 

 medusa or a sessile medusoid individual or gonophore which remains 

 attached to the parent hydroid. In the latter case it may have retained 

 the general form of a medusa or it may be reduced to the form of a bud 

 (sporosac) and have lost all semblance of the medusoid form. The 

 embryo is ciliated and is called the planula: after a period of free life 

 it attaches itself and becomes a hydroid polyp. 



Most Hydromedusae live in the sea. Hydra is found in fresh water 

 and is cosmopolitan in its distribution. Cordylophora is a brackish water 

 form which also occurs in fresh water. A few medusae, Microhydra in 

 Pennsylvania and Europe, Craspedacusta (lAmnocodium) in America, 

 Europe and Brazil, Limnocnida in Lake Tanganyika and Holomisis in 

 Trinidad, occur in fresh water. The class contains about 2,000 species, 

 grouped in 7 orders. 



Key to the orders of Hydromedusae: 



A. THE HYDROIDS. 



d Animals mostly in fresh water 1. HYDBABIAE 



2 Animals marine (rarely in fresh water). 



&! Colony forms a coral-like stock 2. HYDROCORALLINAE 



6 2 Colony not coral-like. 



Ci Colony and individual hydroid usually not minute. 

 d v No protective cup (hydrotheca) on hydranth (Fig. 150) . .3. TUBULARTAE 



d 2 Hydrotheca present (Fig. 172) 4. CAMPANULARIAE 



c 2 Colony and hydroid minute (when present) 5. TRACHOMEDUSAE 



c 3 Colony free swimming , 7. SIPHONOPHORA 



B. THE MEDUSAE. 



(Excluding the Hydrocorallinae and the Siphonophora.) 



0! Rim of umbrella not scalloped. 



&! Gonads on manubrium 3. TUBULARIAE (Anthomedusae) 



6 2 Gonads on subumbrella (sometimes also on manubrium). 

 d Medusae of ten disc-like at maturity. . .4. CAMPANULARIAE (Leptomedusae) 



c 2 Medusa usually hemispherical or elongate 5. TRACHOMEDUSAE 



o 2 Rim of umbrella scalloped 6. NARCOMEDUSAE 



