144 VAN beneden's researches into the dicyemid^. 



of closely united cells, which live together and form a second- 

 ary aggregate. No differentiated organ or internal cavity- 

 is present, so that they are distinctly of a low^er grade of 

 organization than the Metazoa, while the fact of their being 

 multicellular raises them above all the Protozoa. They are 

 best placed, therefore, in an intermediate position, under a new 

 group, the " Mesozoa," which may be characterised as 

 multicellular organisms, composed of two kinds of cells, one 

 layer of which is external or peripheral, performs all the func- 

 tions of animal life, and is a true ectoderm ; whilst the second 

 layer, being the internal or central, performs the vegetable 

 functions and constitutes the endoderra. No vestige of a 

 third cell-layer is present, nor is there any connective tissue, 

 coelom, vessels, muscular fibres, or nerves. 



If the development of the Dicyemidse he taken into con- 

 sideration, we should also be inclined to place them under 

 the group of the Mesozoa, for the gastrula is epibolic, the 

 endoderm is represented by a single cell, and the completely 

 developed organism is only this gastrula enlarged, with the 

 blastopore closed, and, as before stated, the gastrula of the 

 Dicyemidse is comparable with the gastrula of osseous fish. 



All organisms which form the transitional stages between 

 the Protozoa and the Metazoa must be classed under this 

 new group of Mesozoa. Before the appearance of the first 

 Metazoa, a certain number of cellular persons produced from a 

 single individual, instead of dividing, continued to live together 

 to form the first multicellular organism. The Magosphsera 

 of Haeckel gives us some idea of what might have been the 

 appearance of these multicellular organisms ; the protoplasm 

 was then differentiated either by invagination or delamination 

 into ectosarc and endosarc. If the differentiation had taken 

 place by delamination the resulting organisms would, like 

 Magosphaera, have consisted of a single row of identical 

 cells, arranged in a vesicle or in a sphere, and characterised 

 by a homaxial symmetry ; if the differentiation had taken 

 place by invagination, the symmetry would have been axial, 

 and the cells would have been differentiated at the two 

 poles. The first mode of differentiation is seen during the 

 development of the Geryonidse, whilst the second mode 

 occurs in the large majority of the Metazoa. 



The number of cells invaginated is very variable ; if there 

 are many, the gastrula developes by invagination properly so 

 called ; whilst if there are only a few, an epibolic gastrula is 

 formed. 



Under the Mesozoa the hypothetical Gastra^adoe must be 

 grouped, understanding by the term Gastrsea organisms pos- 



