74 THE ANATOMY OF INVERTEBRATED ANIMALS. 



wall, nor, in most cases, is any trace of it discernible at the 

 end of the systole. Occasionally, the vacuole certainly com- 

 municates with the exterior, and there is some reason to 

 think that such a communication may always exist. The 

 function of these organs is entirely unknown, though it is an 

 obvious conjecture that it may be respiratory or excretory. 



The " nucleus " is a structure which is often wonderfully 

 similar to the nucleus of an histological cell ; but, as its iden- 

 tity with this is not fully made out, it may better be termed 

 "endoplast." It is, usually, a rounded or oval body imbed- 

 ded in the protoplasm, and but slightly different therefrom 

 in either its optical or chemical characters. Generally it be- 

 comes more deeply stained by such coloring-matters as haema- 

 toxylin or carmine, and resists the action of acetic acid better 

 than the surrounding protoplasm. 



In a few Protozoa there are many endoplasts in the sub- 

 stance of the body, and the protoplasm shows some tendency 

 to become partially differentiated into cells. But where, as 

 in the higher Infusoria, the body presents a definite organi- 

 zation, with permanently differentiated constituents, which 

 may be properly termed tissues, these tissues do not result 

 from the metamorphosis of cells, but originate from the pro- 

 toplasm directly by changes of its physical and chemical char- 

 acters. 



Conjugation, followed by the development of germs, which 

 are set free and assume the form of the parent, has been ob- 

 served in several groups of the Protozoa, but it is not yet 

 quite certain how far sexual distinctions are established among 

 these animals. 



I. THE MONERA. 



In these lowest forms of animals the entire living body 

 consists of a particle of gelatinous protoplasm, in which 

 no nucleus, contractile vacuole, or other definite structure, 

 is visible ; and which, at most, presents a separation into 

 an outer, more clear, and denser layer, the ectosarc y and 

 an inner, more granular and fluid matter, the endosarc. The 

 outer layer is the seat of active changes of form, whereby 

 it is produced into pseudopodia, which attain a certain 

 length, and are then retracted, or are effaced by the devel- 

 opment of others from adjacent parts of the body. These 

 pseudopodia are sometimes broad, short lobes ; at others, elon- 

 gated filaments. When lobate, the pseudopodia remain dis- 



