XVI INTEODUCTION. 



the specific determination of an unknown plant, he 

 will as certainly determine the species of the sponge, 

 as he would the species of the plant. 



Among nearly allied species of sponges, as with 

 plants, there is frequently to be found a prominent 

 character, an individual structure or organ that at 

 once distinguishes it from every other, however closely 

 allied it may be in form or colour ; thus Hymeniacidon 

 ficus which cannot be separated with certainty by its 

 form, habit, or colour from H. carnosa or H. suherea, 

 is at once discriminated by the vast number of minute 

 inflato-cylindrical spicula in its dermal membrane. 

 These prominent characters are rarely found in the 

 skeleton or in any of the external characters, but they 

 must be sought for among the auxiliary organs, and 

 especially among the external and internal defensive 

 spicula and the retentive ones also. All these minute 

 organs are remarkably constant to the species, and the 

 presence of one form only is frequently determinative 

 of the species ; in other cases the combination of two 

 or more of these forms lead us to the same results. 



Viewing the spongiadee as a whole they have every 

 appearance of being a separate and especial creation, 

 a peculiar class of creation distinct from every other 

 living group, but combining within themselves all the 

 strongly contrasted variety of form and structure that 

 are so strikingly exhibited throughout the whole 

 extent of the vegetable and the animal kingdoms. 

 The forms and varieties of skeleton structure are as 

 numerous and eccentric, if I may be allowed the term, 

 as those of the whole of the higher animals ; they vary 

 in the earthy bases of their skeleton from calcareous 

 to siliceous matter, intermixed with keratode (or 

 cartilage) or to the possession of a keratose skeleton 

 without the admixture of either earth, and in the 

 auxihary portions of their solid structure, their 

 spicula rival the leaves of plants in the extreme 

 variety of their forms, and yet amidst all this amazing 

 multitude of varied forms each one can b^ a practised 



