88 REPRODUCTION OF POLYPES. PART in. 



tive juices, which are supposed to perform the part of a 

 liver. The liquid which circulates in these animals is 

 colourless, with solid particles floating in it ; and there 

 is reason to believe that sea- water is admitted into the 

 tubes, and that, mixed with the juices prepared by the 

 polypes, it circulates through the ramified cavities, is 

 sent into the hollow prehensile tentacles, and returns 

 back into the digesting cavity after having contributed 

 to respiration by its oxygen. The movements of this 

 fluid appear to depend upon the delicate ciliated fibre 

 which lines the cavities of the tentacles and those of the 

 stem and branches of the compound animal, possibly 

 aided by vital contraction. The soft skin of the ten- 

 tacles contains cells full of liquid, with a thread and its 

 sting or dart coiled up within it. These thread stings 

 are protruded when the skin is irritated, which fre- 

 quently gives the tentacles the appearance of being 

 beset with bristled warts. In many instances these 

 kinds of Hydrozoa are covered with a gelatinous sub- 

 stance, either as a film or thick coat. 



The reproduction of many of these arborescent or com- 

 pound Hydrozoa is one of the most unexpected and ex- 

 traordinary phenomena in the life-history of the animal 

 creation. For besides the system of consecutive budding 

 from a single polype which builds up the compound 

 animal, peculiar buds are formed and developed, which 

 bear no resemblance whatever to the polype buds : on 

 the contrary, when mature, they assume an organiza- 

 tion exactly the same as that of the common jelly-fish 

 or Medusoid Acalephse, and swim freely away from their 

 fixed parent as soon as they are detached. These medu- 

 siform zooids, which are extremely small, consist of a 

 cup or umbrella-shaped bell of colourless transparent 

 matter, which is their swimming apparatus ; it is con- 

 tracted and expanded by a muscular band under the 

 rim, the water is alternately imbibed and forcibly ejected, 



