624 SUMMABY OF CURRENT RESEARCHES RELATING TO 



that, further, the ectoderm never gives any indication of the develop- 

 ment of an intermediate layer. In examining the question of the 

 homology of the medullary body of Mliopalura or Dicyema with the 

 endoderm of a Metazoon, it is shown that in their earlier stages of de- 

 velopment all the forms agree closely with one another, and if we 

 allow that the inner layer (endoderm) is homologous throughout the 

 whole of the Metazoic series, we can find no good reason for shutting 

 out the endoderm of the Mesozoa. Certainly no supporting fact is to 

 be found in the absence of a digestive cavity, as the Acoela, among 

 the Ehabdoccela, with their intracellular mode of digestion, are alone 

 sufficient to demonstrate. 



Orthonectida.* — C. Julin here gives a full account of his re- 

 searches, to the preliminary notice of which we have already referred 

 (p. 511) ; he is of opinion that the Orthonectida cannot be considered 

 as triploblastic forms or Metazoa, but as Mesozoa or diploblastic 

 forms, the muscular layer which is developed not being a formation 

 of a mesoderm, but only a histological differentiation of the endo- 

 dermic cell ; so far they resemble the Actinozoa, but they are dis- 

 tinguished from them by the fact that the muscular cells do not, 

 throughout the whole of their life, remain connected with the endo- 

 derm. When we compare them with the Dicyemidaa we find various 

 points of resemblance, (1) Just as in them there are nematogenous 

 and rhombogenous forms, so in these there is a flattened form which 

 gives rise only to female embryos, and a cylindrical form whence 

 develope the males, and the two female forms are to be found in the 

 same host ; (2) the gastrula in both groups is formed by epiboly ; 

 (3) the structure is essentially the same in the two — a ciliated ecto- 

 derm, differentiated into a cephalic region at the anterior pole, and 

 an endoderm of one or of several cells ; the difference in this last 

 character is due to the different mode of formation of the germs; in 

 the Orthonectida they arise by division of the primitive endodermic 

 cell, and in the Dicyemida by an endogenous process. The views of 

 Giard and Metschnikof that the Orthonectida may be compared to the 

 Turbellaria, are objected to chiefly on the ground of the great 

 development of the mesenchyma in these lowly worms ; the doctrine 

 that they have degenerated under the influence of the parasitic habit 

 is not confirmed by the history of their development, in which they 

 exhibit no traces of any higher organization. 



Julin regards the Mesozoa as pluricellular organisms, composed 

 of two kind of cells, ectodermal and endodermal, without any trace of 

 any mesenchyma, ccelom, vessels, muscles or nervous tissue of meso- 

 dermic origin ; they are developed by the division of the egg-cell and 

 its differentiation into a peripheral and a central layer. He insists 

 on the incorrectness of the use of the term metamere or segment as 

 applied to these forms, pointing out that the appearance which has 

 induced some authors to apply these expressions is due only to the 

 presence at certain points of shallow grooves between the transverse 

 rows of ectodermic cells, there is no internal segmentation corre- 



* Arch, de Biol., iii. (1882) pp. 1-54 (3 pis.). 



