SPONGES 63 



to be endoderm, and all non-epithelial tissues to be mesoderm. This 

 view, which for twenty years has been dominant, has in many respects 

 retarded our knowledge of the group, especially from the physiological 

 point of view, since it has led to cells of very diverse nature being 

 lumped together as mesoderm (see below, p. 85). 



We reject here the mesoderm theory, both on structural grounds, 

 which have already been explained (p. 51), and for further developmental 

 reasons ; the fact, namely, that the so-called mesoderm, with the sole 

 exception of the wandering cells, does not represent a primary germ layer 

 set apart once and for all in the embryo, but only a progressively special- 

 ised, and somewhat heterogeneous, portion of such a layer, which, in 

 Calcarea, as already stated, is continually recruited from the dermal 

 epithelium by immigration of cells. The view here adopted is nearer 

 to that of Haeckel ; sponges consist of a dermal layer (not a syncytium) 

 and a gastral layer, together with a number of archaeocytes, not recognised 

 by Haeckel. The homologies of these layers with those of other animals 

 are questions which require special consideration. 



3. Reproduction and Development. 



In sponges generally three modes of reproduction may be dis- 

 tinguished. The first of these may be termed vegetative reproduction, 

 and can only be distinguished from ordinary growth by its leading 

 to the formation of new individuals by budding instead of to a 

 simple increase in size in an individual already existing. The other 

 two methods are effected by means of special reproductive cells 

 (tokocytes), and may be distinguished as asexual, by means of 

 gemmules or special reproductive bodies, and sexual, by means of 

 ova and spermatozoa. The first and third of these methods are 

 seldom absent, the second is less common. 



(a) Vegetative Reproduction. At the outset a distinction must 

 be drawn between cases where the new individuals produced are set 

 free (discontinuous budding), and where they are not (continuous 

 budding). In the latter case the budding is in many cases difficult 

 to distinguish from simple growth, and the distinction between the 

 two processes will depend on the criterion adopted of individuality in 

 the sponge organism (see below, p. 89). If the criterion taken be the 

 embryological one, and each osculum be reckoned as the sign of an 

 individual or sponge person, then the formation of a new osculum 

 in a sponge colony may be regarded as a case of budding, which 

 results in the addition of a new person to the colony. In some 

 cases where the persons, in this sense, are distinct and well 

 individualised, the term budding may be well applied, but in other 

 cases the distinction between growth and budding becomes rather 

 artificial. 



Continuous budding, as above defined, is of almost universal 

 occurrence amongst sponges, except in forms with well-marked 



