26 ZOOLOGY. 
Class 1. Colenterata. 
This class contains the orders Hydroida, Pulmonigrada, Ciliograda, 
Cirrigrada, Physograda, and Diphyida, all of which (excepting the first) 
constitute the Acalephz of Cuvier. For the sake of uniformity in the 
nomenclature, we here propose the name Systoligrada, instead of fpgaaa:, 
the locomotion being similar to that of the Pulmonigrada. 
Orver 1. Hyproma. _ The order Hydroida contains animals, some of 
which have, and some have not a corallum, or the stony material named 
coral. This does not constitute an essential distinction here, or among the | 
Zoophyta, because the condition of the hard material is different in different - 
genera, being sometimes merely indicated by the presence of calcareous 
granules scattered through the body. 
In the Hydroida the internal cavity is tubular and simple, and the order 
includes the four families, Hydride, Sertularide, Campanularide, and 
Tubularidee. 
The Hydride, of which the fresh water polypus is a familiar example, 
occur in-America as well as in Europe. It forms the genus Hydra, and 
is a soft naked polyp (pl. 75, jigs. 21-23) found attached to plants in 
stagnant water, and increasing by lateral buds, as represented in jigs. 22, 
23; but unlike some Zoophyta, the young thus produced become detached 
when they attain a certain size. The internal cavity of the young is for 
some time continuous with that of the parent, so that the nourishment taken 
by the latter can be digested by both. Finally the young gets arms of its 
own, the cavity closes below, and the new animal becomes detached and 
commences an independent life. This mode of increase takes place in 
summer. Trembley, by watching an individual, found that it produced 
forty-five young in two months. 
This genus was discovered in Europe by Trembley, in the year 1739, and 
we have observed it here. It attaches itself by the base to plants, rubbish, 
or even aquatic insects. These animals move somewhat in the manner of 
a leech, by stretching out the body and attaching the arms to an object, 
then drawing up and attaching the base, and so on in succession. When 
placed in a glass of water, they are said to pass from the shade into the 
light. The figures represent them about the natural size, so that they can 
be pretty readily detected with the naked eye. When disturbed, the arms 
and body are contracted into a small compass. The arms are used to catch 
their prey, which consists of minute Crustacea, and other animal food. The 
body is usually sufficiently translucent to allow the contents of the internal 
cavity to be seen. 
The structure of the Hydra is of the simplest kind, pete limited to the 
tubular body and its single aperture for the admission ou exclusion of food ; 
and the margin of aed opening is fringed with from six to eighteen a 
elastic, flexible, and thread-like arms. 
Under a high magnifying power, the arms of Hydra are seen to be studded 
with hemispherical projections, which resemble a bunch of grapes when 
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