RESEARCHES INTO THE DICYEMID^. 143 



of the cells which compose it, for after its birth it does not 

 form a single new cell. The number of cells in the body 

 is twenty-six in the Nematogenous individuals of Dicyema 

 typus and Dicyemina Kollikeriana. A Dicyema is a per- 

 manent gastrula person, whose endoderm is composed of 

 a single cell. 



The germs of Rhombogenous Dicyemidse form endogenously 

 in special cells situated in the axial cell. These creative cells 

 have been called germ producers or germigens, and only a small 

 number of them exist in the axial cell. Each germ producer 

 forms a number of successive generations of germs, which are 

 characterised by their size, by their granular appearance, and 

 by the dimensions of their nucleus. After true cleavage has 

 occurred a small sphere is formed at the expense of each 

 germ; these spheres, which ultimately become the infusori- 

 form embryos, are composed of a certain number of cells, 

 some of which are large whilst the rest are small. The in- 

 fusoriform embryo consists of three parts — an urn, a ciliary 

 body, and two refractive bodies which are united to form a 

 single organ. The embryo so constituted is bilaterally 

 symmetrical. The urn, placed at the side of the belly, 

 consists of a cap, of a cover, and of a contents ; it is com- 

 posed of four granular bodies, each of which encloses several 

 cell-nuclei, and they become ciliated when they have attained 

 their full development. The two refractive bodies are pro- 

 duced in two neighbouring cells, they partially cover the 

 urn in front and form the main part of the dorsal face of the 

 embryo. The ciliary body composes the caudal part of the 

 pyriform embryo, and is formed of a fixed number of ciliated 

 cells. The vermiform embryo developes in the Cephalopod 

 in which it is born, whilst the infusoriform embryo probably 

 distributes the species by transmitting the parasite from one 

 cephalopod to another. 



Relationships of the Dicyemidce. — The Dicyemidse are separ- 

 ated from the Protozoa, even to a casual observer by the fact 

 that they are multicellular ; but they have no third cell- 

 layer, so that they can scarcely be admitted into the group 

 of the Metazoa without some essential modification of the 

 definition which is ordinarily given of this group. Hitherto 

 the Dicyemidae have for various reasons been classed as 

 Vermes by such writers as Kolliker, Von Siebold, G. 

 Wagener, and Ray Lankester ; but even in this heterogeneous 

 group there is no type which offers any analogy to the 

 parasites of which we are treating. The organization, too, of 

 the Dicyemidae is much simpler than that of any of the 

 known Metazoa, for they are composed of a very small number 



