36 FREE AND FIXED ANIMALS 



the nineteenth. Sponges were distinguished from 

 the other zoophytes by the absence of polyps, but 

 at an early stage of modern investigation the 

 discovery was made that currents of water enter 

 the body of the sponge through small pores in 

 its outer wall, and leave it through larger apertures 

 called oscula ; subsequently the free-swimming 

 ciliated larva was made known. In the same 

 way the structure and life-history of other fixed 

 or sessile animals were described, and it became 

 an axiom that all such animals have been derived 

 from free-living ancestors. 



True sessile animals have been defined as 

 those which, during the greater portion of their 

 lives, are unable to transport themselves from 

 the spot to which they have become attached, 

 but, unlike parasites, are able to secure their 

 own sustenance. Each phylum, with the excep- 

 tion of the craniate Vertebrata, has sedentary 

 members ; all sponges, most actinians (corals and 

 sea anemones), and hydroids are sessile ; com- 

 paratively few echinoderms are fixed at the 

 present age, but "the combined evidence of 

 comparative anatomy, embryology, and palaeon- 

 tology indicates that the Echinoderma owe most 

 of their obvious characters ... to their having 

 passed through a pelmatozoic (stalked) stage, 

 i.e., a stage in which the animal was attached 

 by a part of its body wall." 1 The recent and 



1 F. A. Bather, "Echinoderma in Lankester's Treatise on 

 Zoology," part iii., 1900, p. 3. 



