2 SPONGES 



the globe, wherever suitable conditions are to be found. The sponge 

 faunas of the present day are remarkable not only for the abundance 

 and the wide distribution of particular forms, but also for the 

 bewildering variety of species, genera, families, and orders ; and 

 these s3-stematic categories are often defined, on the one hand, by 

 characters of apparently slight and trivial importance ; and con- 

 nected, on the other hand, by numerous intermediate forms, to which 

 it is difficult to assign a definite position in the system. Hence, 

 while the classification of sponges frequently presents great difficulties, 

 at the same time there is perhaps no group which illustrates so 

 strikingly the theory of evolution and descent. Moreover, to judge 

 from the very large number and variety of fossil forms occurring in 

 strata of every horizon, sponges seem to have been at all times 

 equally abundant and widely spread, equally plastic and adaptable, 

 from the earliest geological ages to the present epoch. In contrast 

 with the extreme difficulty often encountered in defining and 

 separating the subdivisions of the Porifera, there is no group of 

 organisms wliich, taken as a whole, is more easily recognised or 

 more sharply limited, both by reason of its peculiar features of 

 organisation and from the entire absence of forms in any way inter- 

 mediate between sponges and other forms of life. Hence it is not 

 surprising that the systematic j^osition of sponges always has been, 

 and still is, much disputed. Even their animal nature was not 

 definitely determined till the middle of this century, and at the 

 present day there is much ditterence of opinion as to their true 

 affinities and proper position within the animal kingdom. These 

 are questions of which the consideration must be deferred until the 

 organisation and development have been discussed. 



From the point of view of the student of animal structure and 

 functions sponges offer many points of interest, as representing 

 the simplest type of cell republic found in any animals above the 

 Protozoa. Their organs are, for the most part, single cells, less 

 specialised than in other forms, and therefore able to perform a 

 variety of functions, either simultaneously or at different times. 

 The absence, or at least the slight degree, of co-ordination between 

 their cells represents a primitive grade of organisation which other 

 Metazoa have passed beyond. Hence many problems of histology 

 and cellular physiology are here presented in their simplest form. 



n. The Morphology and Life-History of Sponges. 



1. External Characters. 



(a) Mode of Attachment. — No sponge is known which, in the 

 adult state, is possessed 'of locomotor organs, or has any power of 

 free movement. After passing through a transitory larval stiige, 



