THE MICROSCOPE IN ZOOLOGY. 167 



by a rhythmical contraction and dilation, as in Rhizistoma, 

 etc. 2. Cilograda, moving by narrow bands of vibratile 

 cilia variously disposed over the body. In Beroe the cilia 

 are transformed into flat fin-like shutters, arranged in 

 eight longitudinal bands. InVenns's girdle, Cestum Ve- 

 neris, the margins of a gelatinous ribbon are fringed with 

 cilia. 3. Physograda, which move by means of an expan- 

 sile bladder, as the Physalia^ or Portuguese man of war. 

 4. Cirrigrada, possessing a sort of cartilaginous skeleton, 

 and furnished with appendages called cirri, serving as oars 

 and for prehension, as Porpita and Velella. In the latter 

 there is also a subcartilaginous plate rising at right angles 

 from the surface supporting a delicate membrane, which 

 acts as a sail. 



This classification has been laid aside since the micro- 

 scopic discovery of the close relationship between the 

 Hydrozoa and the Medusoid Acalephs, and the latter are 

 now subdivided into the " naked-eyed " and the " covered- 

 eyed " Acalephs. The alternation of generations, page 

 126, is fully illustrated in this class. The embryo emerges 

 as a ciliated gemmule, resembling one of the Infusoria. 

 One end contracts and attaches itself so as to form a foot, 

 while the other enlarges and becomes a mouth, from which 

 four tubercles sprout and become tentacles. Thus a Hy- 

 dra-like polyp is formed, which acquires additional tenta- 

 cles. From such a polyp many colonies may rise by gem- 

 mation or budding, but after a time the polyp becomes 

 elongated, and constricted below the mouth. The con- 

 stricted part gives origin to other tentacles, while similar 

 constrictions are repeated round the lower parts of the 

 body, so as to divide it into a series of saucer-like disks, 

 which are successively detached and become Medusae 

 (Plate XV, Figs. 125, 126). 



VIII. ECHINODERMS. This class includes the star-fishes, 

 the sea-urchins or sea-eggs, the sea-slugs, and the crinoids 

 or stone lilies of former ages. If we imagine a polyp with 



