60 ZOOLOGY. 
sarc) forms around it, tentacles arise around the mouth, 
finally the stem branches, new Hydroids arise, until a hy- 
droid community (consisting of ¢rophosomes and gonosomes) 
is formed, and in the following spring medusa-buds (gono- 
phores) arise, which become free (medusoids), and thus the 
reproductive cycle is completed. The developmental his- 
tory of this Hydroid is a good example of what is called 
‘‘ alternation of generations.”’ 
Budding occurs in the medusa of Sarsia prolifera, in 
Hybocodon prolifer and Dysmorphosa fulgurans. Mul- 
tiplication by fission has been observed in the medusa of 
Stomobrachium mirabile. The pendent stomach was seen 
by Koélliker to divide in two, becoming doubled, which act 
was followed by a vertical division of the umbrella, separat- 
ing the animal into two independent halves. These again 
subdivided, and Kélliker thinks this process went on still 
further. Haeckel has found in cutting off a portion of the 
edges of the umbrella of certain Thaumantia, that the frag- 
ment in a few days became a complete medusa. 
In the Tubularian Hydroids (Tubularia, Hybocodon, Co- 
rymorpha, Monocaulus, etc., Fig. 41), 
the mode of reproduction is peculiar. 
From the medusa-buds (sporosac) is set 
free an embryo (actinula), which swims 
about or creeps on its tentacles, mouth 
downward. It then attaches itself by a. 
disk-like expansion of the posterior end, 
which forms a stem until the original 
Tubularia form is attained. 
A gigantic Monocaulus having sessile 
ovisacs, measuring seven feet four inches. 
in height, and provided with a crown of 
tentacles nine inches across from tip to 
tip of the expanded, non-retractile ten- 
Fig. 41.—Monoraulus pen- tacles, was dredged by the Challenger 
wue—Atter Agassiz. Fxnedition at the depth of four miles, 
Allman suggests that such a deep-sea Hydroid could not, on 
account of the darkness and pressure of the water at such a 
great depth, produce free-swimming meduse. In Tiaropsis 
