PORIFERA, CHELENTERATA, VERTEBRATA 91 
“In the hydroid polyps and their medusoids the 
germ-cells always arise in the ectoderm; in species 
which produce sexual medusoids by budding, the 
germ cells arise in the ectoderm of the manubrium 
of these medusoids (Fig. 29, M, kz). But in many 
species these sexual stages have degenerated in the 
course of phylogeny into so-called gonophores, 
that is, to medusoids which still exhibit more or less 
complete bells, but neither mouth (m) nor marginal 
tentacles (7), and which no longer break away 
from the colony to swim freely about, to feed in- 
dependently, and to produce and ripen germ-cells. 
The degeneration of the ‘gonophores’ often goes 
even farther; in many the medusoid bell is repre- 
sented only by a thin layer of cells, and in some even 
this token of descent from medusoid ancestry is 
absent, and they are mere single-layered closed 
brood-sacs (Fig. 30, Gph). 
“The adherence of the sexual animal to the hydroid 
colony has, however, made a more rapid ripening of 
the germ-cells possible, and nature has taken advan- 
tage of this possibility mm all cases known to me, for 
the germ-cells no longer arise in the manubrium of 
the mature degenerate medusoid, that is, of the 
gonophore, but earlier, before the bud which becomes 
a gonophore possesses a manubrium. The birth- 
place of the germ-cells is thus shifted back from the 
manubrium of the medusoid to the young gonophore- 
bud (Fig. 29, M, kz). The same thing occurs in 
species in which the medusoids are liberated, but live 
only for a short time, for instance, in the genus 
