26 ZOOLOGY. 



Class 1. Colenterata. 



This class contains the orders Ilydroida, Puhnonigrada, Ciliograda, 

 Cirrigrada, Physograda, and Diphyida, all of which (excepting the first) 

 constitute the Acalephaj of Cuvier. For the sake of nniformity in the 

 nomenclature, we here propose the name SystoUgmda^ instead of Biphjida^ 

 the locomotion being similar to that of the Pulmonigrada. 



Order 1. Hydroida. The order Ilydroida contains animals, some of 

 wdiich 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 Ilydroida the internal cavity is tubular and simple, and the order 

 includes the four fara'dies^ Hydridse, Sertularidae, Campanularidce, and 

 Tubularidiie. 



The Hydridce., 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 {j^l- 75, figs. 21-23) found attached to plants in 

 stagnant water, and increasing by lateral buds, as represented in figs. 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 lattei: 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, being limited to the 

 tubular body and its single aperture for the admission and exclusion of food ; 

 and the margin of this opening is fringed with from six to eighteen very 

 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 

 230 



