344 THE MICROSCOPE. 



The seas which wash our shores swarm with beautiful 

 forms of minute Polypes, having nearly the same organisa- 

 tion as the Hydra, but which are protected by an external 

 horny integument. This peculiar covering sends forth 

 shoots or buds, which are developed into new polypes, thus 

 producing a compound animal; but the exercise of this 

 gemmiparous faculty is prevented by a horny defence 

 from effecting any other change than that of adding to the 

 general size, and to the number of tentacles, prehensile 

 fingers, and digestive sacs ; yet the pattern according to 

 which new polypes, and branches of polypes, are developed, 

 is fixed and determinate in each species, and there conse- 

 quently results a particular form of the whole compound 

 animal, by which the species can be readily recognized. 



In none of the Hydridce does the hardening outer layer 

 extend over the base of the polypes, forming by its detach- 

 ment here from the inner layer, a cup or cell ; and it is 

 by this circumstance that they are fundamentally distin- 

 guished from the next group, Corynidce, the tentacles of 

 which are not arranged in a single transverse series, as in 

 Hydra, but are usually scattered more or less irregularly 

 over the surface of the polype ; while in Tubularia there 

 are two transverse rows of tentacles, an upper shorter, and 

 a lower longer. The reproductive organs, which in Hydra 

 are extremely simple, attain, in many of the Corynidce and 

 Tubulariadas, to the condition of zooids, sometimes be- 

 coming detached, and swimming about freely before dis- 

 charging their products. These free zooids have always 

 the form of a bell or disc, from whose centre a pyriform 

 or oval body either a closed sac or an open-mouthed 

 polype is suspended; and they have been regarded as 

 distinct animals, and grouped together with other forms of 

 Hydrozoa, under the head of Medusce. The bell possesses 

 considerable contractility, and at each contraction the 

 water which fills its cavity is forced out, and the bell itself 

 is thereby propelled in the opposite direction. In Tubu- 

 laria a more completely medusiform body is developed, 

 but is never detached. 



In the Sertulariadce, the numerous digestive zooids, or 

 polypes, developed from the original germ, remain always 

 attached; but their most remarkable feature is the posses- 



