of tlie Spoiuje Organism. 401 



histologically. The former constitute the covering in the 

 region wiiere the body of the sponge is bathed by the water, 

 but the latter lie inside a connective tissue, and hence cannot 

 be termed ectoderm, so far as regards the adult differentiated 

 sponge. 



It is somewhat different if we extend our investigation and 

 raise the question whether in the course of the phyletic deve- 

 lopment of sponges the covering epithelium cells and the 

 contractile elements were not one and the same, and whether 

 we do not find even now a similar relation in primitive sponges, 

 as E. A. Minchin has done *. He has described in the oscula 

 of a calcareous sponge generally held to be of primitive and 

 simple structure a sphincter which contracts these openings so 

 readily that they have hitherto not been seen. This sphincter 

 consists of two epithelial layers of flat, spindle-shaped, ecto- 

 derm cells ; of mesodermal elements there are only wandering- 

 cells to be observed here and there; hence Mincliin concludes 

 with reason that here the very energetic contraction is only 

 brought about by the ectoderm. Minchin yields indeed too 

 much to Topsent when he subsequently expresses the opinion 

 that all authors have called the muscle mesodermal; but he 

 applies Topsent's and his own results in a more correct manner 

 when he says " that in a highly specialized sponge muscular 

 cells wliich originally formed a part of an epithelium became 

 more specialized and sank into the mesoderm." 



I find the most developed condition in this respect in the 

 liorny sponges, according to F. E. Schulze's well-known 

 description. There " contractile fibre-cells," easily recog- 

 nizable by their structure, lie in great quantities in the 

 mesoderm, i. e. in the connective-tissue mass, often arranged 

 in strands and sometimes forming complete concentric layers 

 " round the canals ; on the other hand, the covering layer also 

 has become further differentiated, the cells of the epidermis 

 liave secreted a fine cuticle, and as far as this extends their 

 power of contraction must be at an end. Thus we have here 

 the division of labour carried to its fullest extent. 



On the contrary, we have before us, not only in the 

 sphincter but in the whole structure of Leucosolenia clathrusj 

 a more primitive condition, as Minchin's histological disco- 

 veries show t' The sponge itself has great power of contrac- 

 tion, and the different forms that have thus arisen were earlier 

 regarded as varieties and then as stages of development. In 



* E. A. Minchin, " Oscula and Anatomy of Leucosolenia clathrus,^' 

 Quart. Journ. Micr. Sci. xxxiii. p. 4 (June 1892). 



t E. A. Minchin, " Some I'oints in the Histology of Leucosolenia 

 clai/irus,'^ Zool. Anzeiger, I8d-, no. o'Ji. 



