. Text-book of zoology for schools and colleges. Zoology. DIVISIONS OF THE HYDKOZOA. 63 is entirely devoted to the duty of providing food for the col- ony, and in these no reproductive organs are ever developed. These nutritive zooids are all like each other in form, and the whole assemblage of them has been appropriately termed the " trophosome " (Allman), from the Greek trepho, I nourish; and soma, body. The colony or trophosome thus formed by the nutritive zooids can go on increasing by the production of fresh zooids for an almost indefinite period; but in all cases there ultimate
. Text-book of zoology for schools and colleges. Zoology. DIVISIONS OF THE HYDKOZOA. 63 is entirely devoted to the duty of providing food for the col- ony, and in these no reproductive organs are ever developed. These nutritive zooids are all like each other in form, and the whole assemblage of them has been appropriately termed the " trophosome " (Allman), from the Greek trepho, I nourish; and soma, body. The colony or trophosome thus formed by the nutritive zooids can go on increasing by the production of fresh zooids for an almost indefinite period; but in all cases there ultimately comes a time when it becomes necessary to produce the essential elements of reproduction in order to secure the perpetuation of the species. The nutritive zooids, as just stated, cannot produce the ova and sperm-cells, being destitute of reproductive organs, and the colony is therefore compelled to produce a second set of buds, which have the power of producing the essential elements of reproduction. These buds are collectively called the " gonosome" (Gr. gonos, offspring; and soma, body). The generative buds have the further peculiarity that not only can they produce the generative elements, but they are altogether unlike the nutritive zooids in appearance. This difference in external appearance and in structure is sometimes so great as to lead to a most remarkable series of phenomena. In the simplest form in which these generative buds or " gonophores " appear, they have the form of mere protuberances of the ectoderm and endoderm (Fig. 16, a), enclosing a cavity derived from the body-cavity. In these buds the generative elements— ova and spermatozoa—are developed (Fig. 15, V). In other instances, the generative buds have a more complicated struct-. FiG. 16.—Generattve buds or gonophores of the JJyWrozoa diagrammatically represented, a Simple gonophore, consisting merely of a protuberance of the ectoderm and endo- derm ; c Gonophore which has the s
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
