8i8 THE ANIMAL KINGDOM. 



an acquired mode of life, and it invariably produces a change in the parasite, which 

 is shown either by the simplification of some structures which have been evidently 

 well developed in an ancestor, or by their total loss in the course of develop- 

 ment. The gastrula of both Orthonectida and Rhombozoa is epibolic, a form which 

 is usually not considered primitive. The most difficult point, however, is the 

 question what character is to be assigned (i) to the 'intermediate endodermic cell 

 masses ' of the male Orthonectid and to the superficial layer of endoderm cells in the 

 female respectively which give origin to muscular fibrils ; and (2) to the two primi- 

 tive germ-cells contained within the parent endoderm-cell of a Dicyemid, one at 

 each pole ? Are these endodermic derivatives to be considered as mesodermic cells 

 originating, as mesodermic cells sometimes do in higher forms, from the endoderm ; 

 or are they to be compared with differentiated ectoderm cells which are still part 

 and parcel of the ectoderm, as occurs with the epithelio-muscle-cells of some 

 Coelenterata ? The Mesozoa will or will not be considered as a rightly established 

 section of the Animal Kingdom, according to which of these two alternatives be 

 chosen. The first points to an origin from some higher form; the second 

 alternative naturally makes in favour of E. van Beneden's views. Further research 

 may throw light on what it must be confessed is no easy problem. 



The questions touched on above are discussed by the authorities cited below. 



Orthonectida^ Julin, Archives de Biologic, iii. 1882; Metschnikoff, Z. W. Z. 

 xxxv. 1 88 1. 



Rhombozoa. Dicyemidae, Whitman, Mitth. Zool. Stat. Naples, iv. 1883 ; E. van 

 Beneden, Bull. Ac. Roy. Belg. (2), xli. xlii. 1876 ; cf. D'Arcy Power, Q. J. M. xvii. 

 1876. Heterocyemidae, E. van Beneden, Archives de Biologic, iii. 1882. 



PROTOZOA. 



Unicellular animals, i. e. animals in which the organism is a single cell 

 physiologically complete in itself. 



An apparent exception to the above given definition is met with in the fusion- 

 plasmodium of Mycetozoa^ or the temporary fused state of some Heliozoa. The 

 compound individual, however, comports itself in every way like a simple one, 

 and there is no differentiation of function. The same statement is true of those 

 colonial Protozoa in which individuals remain in organic connection. The sole 

 example of physiological specialisation among Protozoa appears to be the Flagellate 

 genus Volvox, where the power of reproduction is limited to a few individuals in 

 each colony. 



Notwithstanding their unicellular character, many Protozoa attain a high grade 

 of complexity. The cell may be naked : it may protect itself by a superficial coagu- 

 lated pellicle of protoplasm, or by differentiated cuticular structures. The latter 

 may remain in close connection with the cell, or be completely detached from it. 

 They may be gelatinous, formed of cellulose or a cellulose-like material, chitinoid, 

 or composed mainly of calcium carbonate or silica. In other instances they consist 

 of foreign bodies of very various character held together by a cement organic or in- 

 organic. As a rule, the skeleton is a continuous whole, but in some Heliozoa and 



