INTRODUCTION. 3 



ascertained with tolerable certainty by a cursory 

 examination of the specimen, but the determination of 

 the species can rarely or never be depended upon 

 without a searching anatomical investigation of the 

 structures, nor can this be achieved by the use of low 

 powers of the microscope, because in very many cases 

 the most characteristic elements of discrimination are 

 exceedingly minute, and often also very few in number ; 

 and no definite conclusions can be reached until we have 

 made ourselves intimately acquainted with the whole of 

 the component parts of the subject under examination. 

 Tedious as this mode of thorough investigation may at 

 first sight appear, the admirable adaptation of each 

 part to its own special purpose, and the beautiful forms 

 and arrangement of the spicules and other component 

 elements in the several parts of the Sponge, amply 

 repay the time and care necessary for their investigation 

 and realisation. 



" The striking diversity of form which we observe 

 in a great number of the British Sponges is due both 

 to difference in habitat and varieties of basal attach- 

 ment. The young Sponge, which may have been 

 developed on a slender seaweed or zoophyte, becomes 

 coating and parasitical, but when growing on a shell or 

 stone it assumes either a massive or a coating form. 

 The occurrence of these differences in habitat and 

 localisation render it necessary that they should be 

 observed and recorded. In other branches of marine 

 zoology we have been so accustomed to place depen- 

 dence on form and colour as important characters, which 

 in Sponges are comparatively of very little value, 

 that these cautions become necessary, and more espe- 

 cially so as the figures in Vol. Ill are in many instances 



