THE AXIM-\L KINGDOM 5 



organized colonies of cells. They are so far independent of one another 

 that a sponge, even after being squeezed through cheese cloth, \\ill reor- 

 ganize itself and go on li\'ing. 



Sponges vary from the common Grantia, five to ten millimeters in 

 height and simple in outline, to complex structures larger than a man's 

 head. Nearly all have skeletons, usually calcareous or horny, but siliceous 

 in the glass sponges. The familiar bath sponge is only a skeleton, from 

 which the flesh has been macerated away. Some dozen species of various 

 sizes and textures furnish the commercial product. 



Most sponges are marine, but there are some fresh water species, 

 which develop in the form of thin sheets up to the size of one's hand. 

 There are branched forms, and also colonial. 



WTiatever their size or complexity, most sponges consist essentially 

 of a body-wall that surrounds a central cavity, or cloaca, which communi- 

 cates with the exterior by, at least, one wide opening, the osculum. The 

 bodv-wall consists of an external covering (ectoderm) ; an iimer covering 

 (endoderm j ; and between the two, a thicker covering, or skin (the meso- 

 derm or mesogloea), which secretes the skeleton. There is no ner\'ous 

 system. 



In the body-wall are two sets of radial canals. Those of one set open 

 into the cloaca and are lined with endodermal cells. The others open to 

 the exterior, are lined with ectoderm, and have muscle fibers at their 

 mouths. The two alternate, and are interconnected by smaller passages. 

 The endodermal cells bear flagella, which by their motion set up currents 

 of water inward through the canals to the cloaca, and outward through the 

 osculum. 



Sexes are separate. The eggs are fertilized in the body of the female, 

 and there develop until they become free-s\\imming larvae, which finally 

 settle do-pvTi on the bottom and assume the sessile existence of the adult 

 sponge. 



Since the sponges are a side branch of the phylogenetic tree of the 

 animal kingdom and seem not to have been the ancestors of any higher 

 group, they interest the student of the himian body chiefly as an example 

 of multicellular organization in ver>- simple terms, intermediate between 

 colonial protozoa and organized multicellular individuals. 



Some 2500 li\dng species are recognized, and fossils are somewhat 

 common. 



Phylum 3. COELENTERATA 



The coelenterates include the familiar fresh water hydra and various 

 other pohi^s, solitary and colonial; corals; jellyfishes; and sea-anemones. 

 The Portuguese man-of-war is a colony in which there are several different 

 sorts of individuals, of which some are specialized for swimming and some 



