Vlll INTRODUCTION. 



may be compared with the leaf-buds and flower-buds of 

 the plant. 



There arc two principal classes of them the nutritive, 

 or those which are concerned in obtaining and preparing 

 food for the commonwealth, and the reproductive, which 

 are charged with the propagation of the species. 



The polypite or alimentary zooid, though varying in 

 form and colour, and in the arrangement of the prehensile 

 organs with which it is furnished, is always identical in 

 essential structure with the Hydra, the type of the class 

 Hydrozoa. It consists of a soft contractile body, very mu- 

 table in shape, the walls of which are composed of the same 

 elements as those of the ccenosarc, and are, indeed, a con- 

 tinuation of them. The interior is occupied by the diges- 

 tive cavity, which is not a distinct bag or sac, but a mere 

 hollow scooped out, as it were, in the body-substance. At 

 its upper extremity it terminates in an oral opening ; and 

 below it communicates freely with the general cavity of 

 the ccenosarc, and lies open to the nutrient currents that 

 pervade it. In some families (e. g. Campanulariidce} the 

 base of the stomach is connected with the common canal 

 traversing the stem by a narrow tubular passage, the 

 C( transition-piece " of Reichert *. (Woodcut, fig. i. b.} 



The oral aperture is simple or somewhat lobed, and is 

 commonly borne on the summit of a more or less promi- 

 nent proboscis, which is capable of great elongation and 

 contraction and is remarkable for its mobility. In some 

 genera the proboscis is conical, in others it is trumpet- or 

 funnel-shaped. Amongst the Eudendriida and Campanu- 



* Vide a paper "On the Contractile Substance and Intimate Structure of 

 the C'ampanularies, Sertularia, and Hydride" by Prof. Reichert, Monats- 

 bericht der Akadern. der Wissenschaft. zu Berlin, July 1866. Translated by 

 Dallas, Ann. N. II. for January 1867. 



