THE GKOUP ZOOPHYTA. 



CHAPTER I. 

 THE HYDROZOA, OR HYDROMEDUS^E. 



Tlie Group ZOOPHYTA Class HYDROZOA, OR HYDROMEDUSJE Characters Colonies Reproduction Order 

 CTENOPHORA Characters Venus' Girdle Order DISCOPHORA, MEDUS.E, or JELLY-FISHES Appearance The Disc 

 Method of Reproduction The Lucernarice Order SIPHONOPHORA Characters The "Portuguese Man-of-War" 

 The Calycophone The Physophorse 7 'elella Order HYDROIDA Genus Hydra Characters Gemmation Power of 

 Reparation Sub-order TUBULAHIA Periyonim us Characters Other Tubularians Sub-order CAM PANULAHIA 

 Sertulariidse Plumulariidae Sub-order TRACHOMEDUS^: Order HYDROCORALLINA Milleporidaj Description- 

 Characters Stylasters Other Hydrocorallinse Classification of the Hydrozoa. 



THE Jelly-fish, the Sertularian Polypes, the Hydra, the Sea Anemones, the Alcyonarians, and the 

 Stony Corals are well-known forms of animal life, and their distinctness from the Echinodermata and 

 the other groups already noticed is evident. They constitute the group Zoophyta, and have more or 

 less of a radiate structure, with tentacles ; and there is a digestive cavity within their body, with 

 wide or canal-shaped offshoots from it. The hollow space within the body thus occupied has given 

 them another name Ccelenterata ; * but before this term was applied, the plant-like appearance 

 of many of the group had entitled them to the term Zoophyta. t They are distinct from the group 

 Spongida (Sponges), although some synthetic-minded morphologists classify all together as Coelen- 

 terates. Formerly the name of Polypes, or Polypifera, was given, on account of the tentaculate 

 body. There are two classes of the Zoophyta the Hydrozoa and the Anthozoa. 



THE CLASS HYDROZOA, OR HYDROMEDUS^E. 



A vast number of marine and a few fresh-water animals, popularly called Polypes and Jelly-fish, 

 belong to this class. All are very delicately and beautifully constructed, and they present great 

 varieties of shape and methods of life. The fresh-water Hydra, the pretty feathery Polype-stems on 

 sea-shells and rocks, the Sertularians and Tubularians, the Jelly-fish, the Portuguese Man-of-war, the 

 Beroes, the Stony Millepores of reefs, and the coloured Stylasters of the deep sea, all have certain 

 structures in common, in spite of their diverse shapes and habits. 



The essential parts of these animals are a mouth, leading directly to a cavity which is digestive 

 in its function, and relates to the circulation of a nutritive fluid, an outer delicate skin, or ecto-derm, 

 encasing the body, and an inner, lining the internal cavity and mouth, and the reproductive organs 

 which are outside the stomachal cavity, and are usually in specially modified parts of the body. These 

 last may be simple sac-like projections of the ecto-derm, or they may be complicated, and have an 

 innei-, and also a meso-derm (middle-skin), covered by the ecto-derm, and may resemble ball-shaped 

 Jelly-fish stuck on by their upper part. 



The Hydrozoa have tentacles, some very slender and others comparatively stout, and certain 

 stinging cells called nematocysts. The organs of special sense are in a very rudimentary condition, 

 but the tissues as a rule are highly sensitive to irritation, and are very contractile. Some of the 

 class are free-swimmers, and others are fixed during all or part of their life cycle. Most are soft 

 and easily destroyed, but some have very solid sub-structures. 



The stationary forms are in colonies of individuals, connected by root- like supports, and in some of 

 the free-swimming kinds there is a colony beneath a float as in the Portuguese Man-of-war but the 

 Jelly-fish are solitary. The colonies may be of simple or of branching individuals, some of which are 

 for the purposes of the nutrition and others for the reproduction of the species. In their construction 

 there is an outer and inner derm, and a central cavity reaching from the root-like supports to the 

 mouth. The opening from the outside into the mouth is without a gullet, and the stomach, or 

 somatic cavity, is digestive as well as referable to the circulation, and it may be simple or may be 

 continuous with canals which radiate from it. The reproductive process is very varied. In some 

 free-swimming Jelly-fish the kind is reproduced by the budding of small ones from the region of the 

 mouth, or eggs may be developed and set free, which become like the parents. But these methods 

 * Greek, koilos, hollow ; enteron, bowel. t Greek, zoon, animal ; phyton, plant. 



