I860.] Hixcks on Zoophytes : the History of their Development. 403 



particular species. Throughout this tribe, we meet with the tendency 

 to an indefinite repetition of similar parts, which is characteristic of 

 the vegetable kingdom, and invariably marks a low grade of animal 

 being. 



In the history of the Order to which this paper is more specially 

 devoted, the Hydroida, or Hydroid Zoophytes, there are facts which 

 exhibit a still further parallelism to certain phenomena of veget- 

 able life. The plant has its leaf-buds and its flower-buds : the one 

 connected with the nutritive, the other with the reproductive, func- 

 tions, distinct in structure, form, and position. And the analogues of 

 these exist in the Zoophyte, which not only puts forth its multitude 

 of polypites to secure and prepare food for the commonwealth, but 

 also originates another class of buds, wherein the reproductive bodies 

 are matured, which perpetuate and diffuse the species. 



Whilst, therefore, the two organisms belong to distinct kingdoms 

 of Nature, they are connected not merely by similarity of external 

 aspect, but by a certain analogy of life. And some of the most re- 

 markable and perplexing points in the history of the Hydroida may be 

 best explained, by collating them with the familiar facts of the 

 vegetable world.* 



To gain a clear conception of the structure of the Hydroid Zoo- 

 phytes, we must start from the common Hydra, the simplest element 

 in the life-series to which they belong. In this historic animal, we 

 have an epitome of the essential characters of tbe Order : in its life, 

 we have the story of that Order " writ small." It presents us with a 

 gelatinous body of mutable form, the centre of which is pierced by a 

 cavity extending from end to end of it, that communicates at one 

 extremity with the external world through a simple orifice, while at 

 the opposite pole a somewhat expanded disc fulfils the contrasted 

 functions of attachment and locomotion. The terminal mouth is en- 

 circled by a number of delicate and extensile threads, which bristle 

 with an armature of minute but fatal weapons (thread-cells) and 

 serve at once for the capture and destruction of prey. This simple 

 structure, submitted to analysis, declares itself to be composed of 

 two essential elements, which enter into every portion of it, and of 

 which its few special organs are but modifications. Two membranes, 

 an outer and an inner — the one in relation with the external, the other 

 with the internal world — constitute the material out of which the Hydra, 

 and all the kindred Hydroids, are wrought. The body is, as it were, 

 a double sac, of which the arms that form a wreath round the orifice, 

 are but tubular prolongations. The double membrane is the basis 

 of Hydroid structure. 



The provision for the multiplication of the species is twofold. 

 New individuals are produced in immense abundance by a purely 

 vegetative process. From the surface of the body, just as from the 

 stem of the plant, buds pullulate, and these are rapidly developed 

 into Hydrse, which are perfect copies of the original stock. During 



* In his ' Metamorphoses de l'Homnie et des Animaux,' chapter xxii., M. de 

 Quatrefages has drawn an elaborate parallel between the structure and reproduc- 

 tive history of the plant and the Zoophyte. 



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