48 THE ANATOMY OF INVERTEBRATED ANIMALS. 



terior protoplasm as a rounded mass, the "endoplast" or 

 "nucleus." Other Protozoa advance further and acquire 

 permanent locomotive organs. These may be developed 

 only on one part of the surface of the body, which may be 

 modified into a special organ for their support. In some, a 

 pedicle of attachment is formed, and the body may acquire a 

 dense envelope (Infusoria), or secrete an internal skeleton of 

 calcareous or silicious matter (Foraminifera, Padiolaria), or 

 fabricate such a skeleton by gluing together extraneous par- 

 ticles (Foraminifera). 



A mouth and gullet, with an anal aperture, may be formed, 

 and the permeable soft central portion of the protoplasm may 

 be so limited as to give rise to a virtual alimentary tract be- 

 tween these two apertures. The contractile vacuole may be 

 developed into a complicated system of canals (Paramoeci- 

 um), and the endoplast may take on more and more definite- 

 ly the characters of a reproductive organ, that is, may be the 

 focus of origin of germs capable of reproducing the individ- 

 ual ( Vorticella), In fact, rudiments of all the chief system 

 of organs of the higher animals, with the exception, more or 

 less doubtful, of the nervous, are thus sketched out in the 

 Protozoa, just as the organs of the higher plants are sketched 

 out in Caulerpa. 



In the Metazoa, which constitute the rest of the animal 

 kingdom, the animal, in its earliest condition, is a protoplas- 

 mic mass with a nucleus — is, in short, a Protozoon. But it 

 never acquires the morphological complexity of its adult state 

 by the direct metamorphosis of the protoplasmic matter of 

 this nucleated body — the ovum — into the different tissues. 

 On the contrary, the first step in the development of all the 

 Metazoa is the conversion of the single nucleated body into 



an aggregation of such bodies of smaller size — the Morula — 



... 

 by a process of division, which usually takes place with great 



regularity, the ovum dividing first into two segments, which 

 then subdivide, giving rise to four, eight, sixteen, etc., 

 portions, which are the so-called division masses or blasto- 

 meres. 



A similar process takes place in sundry Protozoa and gives 

 rise to a protozoic aggregate, which is strictly comparable to 

 the Morula. But the members of the protozoic aggregate 

 become separate, or at any rate independent existences. 

 What distinguishes the metazoic aggregate is that, though its 

 component blastomeres also retain a certain degree of physi- 

 ological independence, they remain united into one morpho- 



