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 ; 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- 



