MARINE AQUARIA 



Halisarca, having a gelatinous or fleshy substance forming an ir- 

 regular crust on the object to which it adheres. 



HalichondUia, having an elastic body permeated by canals opening 

 to the surface and siliceous spicula in the fibre distributed throughout its 

 homologous body. 



Spongia, having a very porous body composed of net-fibres traversed 

 by canals opening to the surface, the fibres without spicula. Very irregu- 

 lar and variable in form, and parasitical or incrusted on and under stones 

 and other objects. 



Sponges should not be introduced into smaller aquaria and should 

 be carefully scraped from stones and other objects, as they soon decay and 

 may pollute the water. 



CcELENTERATAOR PoLYPS. These are somewhat higher forms ofanlmals. 

 The Hydrozoa are zoophytes resembling seaweeds, which live In colonies, 

 small compound jellyfishes composed of many individuals; and the MIl- 

 lepores live in colonies and secrete a stony instead of horny skeleton. 

 Of these no further mention need be made. The Scyphozoa are larger 

 jellyfishes of various forms of development, but also do not require men- 

 tion. The Actinozoa Include anemones, actinia and actlnoid corals, of 

 which the first are of particular Interest, but other forms of Corals, Cor- 

 allines, Sea-pens, Sea-whips and Sea-fans, and the Ctenophora, including 

 the Comb-jellies, require but brief mention, as none of. them will survive 

 In the Marine aquarium. 



Sea Anemones and Actinia. These polyps belong to the order 

 Actinarla. They mostly inhabit crevices, dark holes and under stones, 

 but some hardy species may be taken in exposed positions, or piles, sub- 

 merged woodwork and other firm objects. There are both deep and 

 shallow-water forms. They generally consist of an undivided column or 

 stalk with a pedal disc or holdfast at the under side, and an oral disc with 

 central mouth surrounded by circles of tentacles at the upper. They are 

 capricious In the forms which they assume and when fully expanded are 

 often of considerable size, but are sensitive and when disturbed collapse 

 and shrink into inconspicuous dusky buttons,, as they are soft and very 

 contractile. The most of the Actinarla are developed from eggs which 

 form on the edges of the inner walls of the column. These, when ma- 

 ture, are ejected from the mouth, small ciliated spheres which swim about 

 until they find a suitable location to which to attach themselves. Some 

 forms reproduce by budding, either on the disc or from^ the sides of the 

 column. As a means of defense they emit long slender threads or 

 acontia, having minute stinging cells. Nearly all are carnivorous and feed 

 on small animal organisms, crustaceans and molluscs. 



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