PREFACE. Vll 



naturalists who may be induced to pursue similar investi- 

 gations, and they will, I trust, find their labour facilitated by 

 the endeavours I have made to systematise the species, and 

 to construct a language of description by which their parts 

 may be known and described by future students of their 

 history. The necessity for this extension of my subject 

 beyond the limits of the British species, becomes the more 

 apparent when we consider that in the larger portion of 

 living creatures our knowledge of them may be greatly 

 facilitated by accurate figures of their external forms and 

 their colour, but we have this assistance to a very slight 

 extent with the Spongiadae. No two specimens of a 

 species agreeing precisely in form with each other, and 

 the discrepancies in shape arising from differences in 

 age, degree in development, and the varied influences of 

 locality, are such as to perfectly bewilder the student who 

 depends on external form as a means of recognition, and to 

 complete his confusion the variations of colour to which 

 many species are subject is almost as great in proportion as 

 that of external form. To these difficulties, perhaps, we 

 may in a great measure attribute the neglect with which 

 this branch of marine natural history has been treated, and 

 the slow progress that has been made in acquiring a know- 

 ledge of them, even by the most enlightened and philo- 

 sophical of the naturalists of the past and present centuries. 

 Their nature is also such as to present scarcely any at- 

 tractive feature to the curious student in zoology. No 

 animal motion, no functional demonstration is visible to the 

 eye of the casual observer to attract his attention from the 

 active and more beautiful tribes of marine animals amidst 

 which they are found, and it is only when we sit down 

 studiously to examine their anatomical structure by the aid 

 of a good microscope that we become aware of the ex. 



