34 The American Naturalist. [January, 
animals are so commonly throwing off from their bodies. If 
the semi-fluid coat of this sort were stiffened only a little, we 
should arrive at the more compact chitinous cuticle of the caly- 
culate forms. The case of Perigonimus thus furnishes a sug- 
gestion of the probable history of the chitinous cuticle of the 
hydroids; at first a thin envelope, later a stiffened cover form- 
ing a greater protection to the body and providing for freedom 
of motion by the development of joints at stated intervals. The 
facts of ontogeny are in favor of such a view of the history of 
the cuticle, for we know that the cuticle arises as an excretion 
thrown off from the ectoderm and hardened on exposure to the 
water. And the differences between the gelatinous and the 
chitinous cuticle, are such differences in the chemical or meta- 
bolic functions of cells as could conceivably easily come within 
the range of operation of natural selection. It is of great 
interest then to find so primitive an animal with so primitive 
a mode of skeleton building, and whether the creature is really 
a primitive one or its primitive characters are only secondarily 
acquired it is one the entire life history of which would be full 
of interest. 
EXPLANATION OF THE FIGURES. 
Fig. 1.—View of general mode of branching and situation of 
medusæ. a hydrorhiza; b. hydroid person ; c. medusa 
bud; d. the gelatinous cov. e. a young hydroid bud. 
Fig. 2 Soa SS view of the terminal zooid showing the body, 
“ec,” “en;” and the covering “thec ;” the proboscis, 
ipio: ” and a younger zooid, “juv.” 
Fig. 3.—View of a single one of the medusæ in position. ° ¢. st, 
the stem of the colony; man. the manubrium inside 
the bell; oc, the eye-spot at the location of a tentacle; 
ten, a tentacle. 
Figs. 4, 5, 6, 7, 8, 9, 10, different successive stages in the growth 
of the iak. 
Fig. 11, a medusa just freed from the colony ; mn. manubrium, 
oc, ocellus ; v. velum. 
Fig. 12, view of an exceptional specimen showing bifurcated 
tentacle. 
Hamline, Minnesota, July 19th., 1893. 
