III. ARTIFICIAL KEY FOR THE DETERMINATION OF 



SPECIMENS. 



I. Entire animal consists of a single cell, though occasionally with 



more than one nucleus; either independent or arranged in the 

 form of a colony , Protozoa. 



II. Animal multicellular, usually of an appreciable size; when 



minute, then almost always with certain definite organs, such 

 as an intestinal canal, ciliated bands, paired limbs, etc. 



A. Structure radiate. 



I. Parts in 2s, 4s, 6s, etc. 



(a) Nettle-cells for defense, placed upon tentacles 

 or other exposed parts; both free-swimming 

 transparent forms (" medusce") and sessile, 

 vase-shaped forms {^'polyps" ), the latter 

 usually colonial. Apical (= ab-oral) pole 

 non-sensitive, serving in sessile forms as 



place of attachment Cnidaxia. 



(i5) Free-swimming, transparent, pelagic forms, 

 with eight meridional rows of swimming- 

 paddles. Apical pole furnished with sense- 

 organs Ctenophora. 



2. Parts in 5s Echinodermata. 



3. Parts not in a definite number, but animal more or 



less vase-shaped or cylindrical and with an apical 

 excurrent opening, and minute incurrent pores 

 along the sides; no tentacles; walls usually thick 

 and their substance filled with a skeletal support 

 of either spicules or of horn threads .... Porif era. 



B. Structure bilateral, or nearly so. 



I. Body externally segmented (or apparently so). 



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