RESEARCHES ON THE DICYEMID^. 133 



found in Octopus vulgaris, and D. Clausiana, in Ociojms 

 macropits. To a second genus, found in the Eledone he gives 

 the name Dicyemella, which is also divided into two species, 

 D. Wageneri, from Eledone moschata, and D. Mulleri, in 

 Eledone cirrosa. A third genus, Dicyeniina, contains the 

 species D. Kollikeriana of the Sepia officinalis, and Dicye- 

 mina Schultziana, from Sepia biserialis; whilst a fourth genus 

 Dicyemopsis, contains the form D. macrocephalus, inhabiting 

 Sepiola Rondeletii. 



Modes of preparation, — Prof. Van Beneden finds that 

 the best method of preserving specimens of Dicyema is to 

 immerse the renal organ of the cephalopod in osmic acid 

 1 per cent, to -j'^ per cent, for from three to ten minutes, then 

 wash and examine in water or very dilute glycerine (1 in 10); 

 for permanent preparations stronger glycerine is used, and 

 they are stained in a mixture of picrocarmin and gly- 

 cerine, but to the use of this staining fluid there is an 

 objection, from the fact that it stains the tissues very slowly 

 after the action of osmic acid, weeks or even months being 

 required before a proper tint is obtained. Very dilute solu- 

 tions of acetic acid (1 in 500 or 800) constitute a very 

 ready dissociator, and bring out the nuclei of the cells very 

 distinctly. Again, the ordinary solution of hsematoxylin 

 prepared with alum gives good results when it is applied 

 directly to the living organism, for it renders the shapes of 

 both cell and nucleus very distinct, though it ultimately 

 spoils them. Absolute alcohol may also be used ; but chromic 

 acid, bichromate of potash and of ammonia, Miiller's fluid, 

 picric acid, and saline solution, are worse than useless, for 

 they break up and destroy the cells. 



The organization. — The body of Dicyema is composed, first, 

 of an enormous axial cell, which is cylindrical or spindle 

 shaped, and extends from the anterior end of the body to the 

 caudal extremity ; secondly, of a single layer of cells form- 

 ing around the axial cell a kind of simple pavement epithe- 

 lium. The single axial cell Mr. Van Beneden calls the 

 endodermic cell, and he considers it homologous with the 

 endoderm (enteron) of the Metazoa; the layer of cells sur- 

 rounding this axial cell is similarly the ectoder-m or ectoder- 

 mic layer (deron) (fig. 3). 



Each cell of the ectoderm has an external or superficial 

 face, which is covered with vibratile cilia, an internal or 

 deep face by which these cells are in contact with the axial 

 cell, and lateral faces by which they are joined to each other 

 (fig. 1). Disposed round the anterior extremity of the axis 

 of the body, which may be called the oral pole, are a series 



