TYPES OF COXSTKUCTIO.Y IX THE ANIMAL KINGDOM. 22$ 



in a transverse direction. The technical name for this mode; 

 of repetition of parts is CDitimcric. 



Distinctive Characters of the Metazoa. — All the higher tissue- 

 bearing animals, or Metazoa, differ from the Protozoa by the 

 possession of the following characters, viz. : 



Metazoa. — Reproduce by developing &%^, or ovum, which 

 passes through the stages of ici) nucleated cell, {b) segmenta- 

 tion, [c) blastosphere or morula, id) gastrula; tissues differen- 

 tiated into {f) ectoderm, {f) endoderm, and [g) mesoderm ; 

 (//) alimentary cavity, or enteron permanent, and (/) sexual 

 differentiation the rule and almost universal. 



Molluscan Type of Structure. — The third type is that in which 

 neither metameric nor antimeric repetition is carried on, but 

 bilateral symmetry and simple antero-posteriority and dorso- 

 ventral polarity are more or less conspicuous. In this type 

 of organisms (the Mollusca) differentiation is expressed in 

 the relative positions of the organs in the body-cavity, and in 

 the relative development or importance of the different or- 

 gans or regions of the body. 



Development of Organs and their Taxonomic Rank and Value. 

 — In the molluscan type is seen in its simplest form that 

 relative development of the several systems of organs which 

 marks the rank of the stage of progress in differentiation in 

 each particular case. Thus of the several systems of organs 

 sustentation is more fundamental, and may be regarded, if 

 prominent in relative development, as indicating primitive 

 or low rank. Organs of correlation, when more specialized 

 and according to the degree of differentiation of the special 

 organs, imply specialization, hence high rank. Thus among 

 the Mollusca those which are simply digestive sacs, with no 

 specialized organs of sense, or of definitive motor organs, are 

 low in rank (the Lamellibranchiata). The specialization of 

 sense-organs anterior to a mouth and of the muscular system 

 for giving definiteness to the motion, indicates higher rank 

 (the Gastropoda and Pteropoda). Special tactile organs, and 

 high development of sense-organs, all in front of the oral 

 opening, show still higher rank (the Cephalopoda). 



This principle of differentiation in the development of 

 organs throws light upon the rank of particular organisms in 



