MESOZOA. 817 



view supported by the observation that it sometimes occurs in a degenerative con- 

 dition in the axial endoderm cell, and by its resemblance (not very close) to a male 

 Orthonectid (supra). 



The vermiform embryo is produced by the division of a germ-cell first into two, 

 then into four, cells. Of the latter, one remains passive, and eventually grows into 

 the axial endoderm cell of the adult ; the other three divide and give origin to the 

 ectoderm cells. At the time of birth the vermiform embryo resembles its parent, 

 and contains germs, sometimes even embryoes. Birth, as in the case of the infusori- 

 form embryo, takes place by simple perforation of the endoderm and ectoderm 

 cells. 



The Heterocyemidae differ from the Dicyemidae by the absence of a calotte, 

 and in some minor points. There are two species, Conocyema polymorphus from 

 Octopus vulgaris, and Microcyema Vespa from Sepia officinalis. 



As to the first-named, the Nematogenous individual varies somewhat in shape. 

 Four apical cells at one extremity (the head) tend to become verruciform, and some- 

 times throw out processes carrying stiff cilia. The remaining ectoderm consists of 

 relatively few cells; their cilia are lost with age, and the cells themselves either 

 flatten out and their limits become indistinguishable, or they may even be lost alto- 

 gether. There is a single vacuolated endoderm cell. The vermiform embryo is 

 formed as in Dicyema, but is conical in shape, the point of the cone being formed 

 solely by the apical cells ; the endoderm cell is spherical. The Rhombogen possess 

 a more or less spherical endoderm cell, and an ectoderm consisting of a very few 

 cells, non-ciliated and capable of throwing out pseudopodia ; by their means the 

 individuals are often united into colonies. The infusoriform embryo resembles that 

 of Dicyema. 



The adult Microcyema Vespa is cylindrical in shape, slightly enlarged at one end, 

 and composed of a single endoderm cell covered by a thin non-ciliated cortical layer 

 (= modified ectoderm?). The embryo in profile resembles closely a Wasp. Its 

 anterior half is truncated, and consists of two ciliated ectoderm cells and a granular 

 mass (? cell or cells), which bears long stout cilia ; its posterior half is fusiform, and 

 consists of two ciliated ectoderm cells inclosing a single endoderm cell. 



It has been asked, is the establishment of a group, Mesozoa, for the reception 

 of the Orthonectida and Rhombozoa justifiable? Do the organisms themselves 

 really possess the exceedingly primitive structure assigned to them, or are traces of 

 simplification of organisation observable ? 



It is self-evident that a great gap intervenes between the most highly differ- 

 entiated unicellular Protozoon and the most lowly Metazoon, with ectoderm and 

 endoderm, and mesoblast or mesoglaea. In every ontogeny a morula or blastula 

 stage, and, except in certain parasites, a gastrula stage of some kind is to be traced ; 

 and it is of course to be concluded that they represent ancestral forms of develop- 

 ment. None such are, however, known to exist as independent and perfected 

 organisms, unless the Orthonectida and Rhombozoa are to be considered as modified 

 gastrulae. The syncytium of Haeckel's Gastreadae (p. 809) is doubtless an ecto- 

 derm plus mesoglaea as it has proved to be in the calcareous sponges : and the 

 disc-like organism, Trichoplax, (Schulze, Z. A. vi. 1883) has a cellular mesoglaea. 

 They are therefore Metazoa. 



The fact that the Mesozoa so-called are parasitic organisms naturally raises a 

 presumption that they are in some way degenerate. Parasitism must in all cases be 



