TYPES OF C@LENTERA—A MEDUSOID. 149 
and elastic ; and (c) the future ectoderm of the adult. In Aydra fusca 
the egg is separated from the parent before the shell is formed, and is 
fastened by its gelatinous sheath to aquatic plants; in A. werddés and 
1. grisea the egg falls off after the outer shell has been formed. In 
all species the separation from the parent appears to be followed by a 
period of quiescence lastirfg from one to two months. It is probable 
that this resting-stage is carried by wind and birds from one water basin 
to another. 
Within the shell differentiation at length recommences, but it pro- 
ceeds slowly. Interstitial cells arise in the ectoderm; a middle 
lamella is formed ; a gastric cavity begins to appear in the midst of the 
endoderm. Thereafter the shell bursts, and development proceeds 
more rapidly. The embryo elongates, acquires a mouth by rupture at 
the distal (sometimes called vegetative) pole. The inner sheath is also 
lost, and the young “ydra fixes itself and begins to live as its parent or 
parents did. 
Forms like Hydra.—Even simpler than Hydra is Protohydra, 
without tentacles, occurring both in the sea and in fresh water. An 
American fresh-water form (Mécrohydra ryder?) is known to liberate 
free-swimming medusoids. A fresh-water Medusoid Limnocodium was 
found in the Victoria Regia tanks in the Botanic Gardens, Regent’s. 
Park, London. Its native habitat is unknown. Another species, 
L. kawaiz, has been found in the Jantszekiang in China, 1000 miles 
from its mouth, A related form, Limnocndda, occurs in Lakes Tangan- 
yika and Victoria Nyanza, and in the river Niger. A strange simple 
polype—Lolypodium—has been found as a parasite on the eggs of 
sturgeons. Further details in regard to all these forms are much 
wanted, 
Second Type of CELENTERA.—A Medusoid. 
Class HypRozoa 
Hydra is too simple to be thoroughly typical of the 
Hydrozoa. The class includes the hydroid colonies or 
zoophytes, which may be compared to Aydre with many 
buds, and also free medusoid forms, which may be (a) 
liberated members of a hydroid colony, or (4) independent 
organisms. Besides these there are complex colonies of 
medusoid forms (Siphonophora). 
The hydroid type, except in minor details, usually 
resembles Hydra. In some cases the tentacles are solid, 
instead of hollow as in Hydra, and they may be arranged in 
two circles,—an outer and an inner (¢.g. Zudularia). In 
some of the hydroid colonies, notably the Millepores and 
Aydractinia, the polyps are very dissimilar to one another, 
and have become specialised for the performance of different 
functions. 
