2 Reviews and Notices of Booh. 



buds which may either remain attached or become free and 

 found new colonies. This gives us the condition of the HydrcB 

 and Tubularioe. Next let us suppose that by budding, such a 

 creature founds a complex branching structure, furnished with 

 little horny cells, in each of which resides an animal contributing 

 its quota to the general nourishment of the whole community. 

 This is the condition of the Sertularlce, Gampanular ice, &c., so 

 abundant in the ocean, and forming so large a part of the zoo- 

 phytes of the older naturalists. Further, these colonies of Hy- 

 droids, conspire to produce capsules, growing like fruits on their 

 tree-hke structures, and giving birth not to young polyps, but 

 to little medusae like the Cyanea, but simpler in structure, and 

 which produce the eggs destined to form new colonies of Hy- 

 droids. It is here that we perceive the connecting link between 

 the Hydroids and the true Medusse. 



But another modification of the hydroid structure is found in 

 nature. Let us suppose that we have taken one of the complex 

 branching structures last mentioned, and have cut off all the 

 tentacles of the polyps, and taken out of the cells all the 

 digestive sacks, and placed by themselves all the reproductive 

 buds or capsules. Let us farther suppose that we have stuck 

 these parts separately, and still alive, upon a living animal film 

 spread over the surface of a shell or stone, or attached to the 

 underside of a hollow gelatinous sack floating on the sea, so as to 

 produce a complex group of separate tentacular, digestive, and 

 reproductive parts, we shall then have the structures of the Hy- 

 dractinia, that clothes some of the dead shells of our cabinets 

 with a rough, unsightly film, and of the Physalia or Portuguese 

 man-of-war, that floats hke a purple and orange bubble on the 

 tropical seas. Such are the general aspects of the Hydroidea. 



In the second order, the, Discophora, already illustrated by 

 the example of the Cyanea^ the animal is greatly expanded lat- 

 erally into a locomotive disk, to the edges of which the tentacles 

 are attached. 



In the third order, the Ctenophora, the parts become fewer but 

 more complex, and the normal form is a transparent gelatinous 

 ball, with the digestive cavity in the centre, the mouth in front, 

 two pulsating vessels for circulating the blood at the sides, loco- 

 motive fins arranofed in bands like the ribs of a melon, two longr 

 and complicated tentacles behind, where are also the eye specks. 

 It is the last order that is specially described and illustrated in 



