THE INVERTEBRATE LEGIONS 93 



Glass Sponges: Class Hexactinellida — Figure 24 



These are mostly vaselike sponges of deep water. The skeleton is formed 

 entirely of six-rayed spicules of silicon compounds. Since most of them must 

 anchor in the mud of deep water, there is commonly a rooting tuft of elongated 

 spicules at the base of the sponge. The skeletons are frequently extremely 

 beautiful glassy objects like that of the Venus flower basket, Eii-plectella. This 

 group is found in deep water of all seas but is most common in the tropics. 



Horny Sponges: Class Demospongiae — Figure 24 



The great majority of sponges belong here, and most sponges encountered by 

 the diver will be of this class. The skeleton is largely formed of spongin fibers, 

 which gives a breadlike texture to most of them. Some of them, however, have 

 siliceous spicules (never six-rayed as- in glass sponges), and a few have no 

 spongin but only have siliceous spicules. Some of the species have echinating 

 spicules, that is, the spicules are sharp and break off in whatever object they 

 touch. The fire sponge, Tedania, and the poison-bun sponge, FihiiUa, are two 

 of these. The fire sponge is shaped like the red sponge Ccolor photograph'), but it 

 is usually a much more brilliant reddish orange in color. The poison-bun 

 sponge is breadlike in texture, brownish in color, and grows in rounded clumps 

 like many common sponges. Both of these are West Indian and capable of 

 producing more or less severe burning stings. Because of the irregular shape 

 of many sponges, it is hard to recognize some of the species, so it is a good 

 precaution to wear heavy gloves when handling them. 



The colors of horny sponges vary widely from the yellow bread-crumb sponge 

 to the brilliant red of the fire sponge and the red-beard sponge, the purplish red 

 of the red sponge, the green of some boring sponges and vase sponges, and the 

 drab brown or black of bath sponges and basket sponges. The form of horny 

 sponges is even more variable. The bread-crumb sponge, Halichondria, and 

 boring sponges, Cliona, which can bore into the shells of molluscs or into rocks, 

 are encrusting; the red-beard, Microciona, and the finger sponge, Chalina, are 

 erect and branching; many are like vases and are colonial (/ig. 25 and color 

 ■photograph); the bath sponges, Euspongia, are rounded; the basket sponge, 

 Hircina (/xg. 25) is cup-shaped and almost large enough to sit in. Almost all 

 of these types have extremely wide distribution, but horny sponges are most 

 varied and are largest in the tropics. The area of the Bahama Islands is 

 especially rich with them. 



POLYPS AND MEDUSAS: Phylum Coelenterata 



These are the jelly animals— jellyfishes, hydrozoans, gorgonians, corals, 

 anemones— so named because of the presence of a jellvlike substance, consisting 

 of about 98 per cent water, which is found between their outer (ectoderm) 

 and inner (endoderm) layer of cells. These are the lowest members of the animal 

 kingdom which have cells organized into organs— digestive, muscular, nervous, 

 reproductive— and they also show, for the first time, a nervous system. This is 

 a very primitive system with no brain and consists only of a network of nerve 



