GLOSSARY. 



form, is an air-breathing animal. Thus while the stages of onto- 

 genetie development can be seen with the eye, the stages ofphy- 

 logenetio development (which, see p. 69, the former are believed 



I to represent) can only be seen with the mind's eye. 



Phylum, pi. phyla (Gk. (j>u\ov, a kind or race). — A name given by 

 some zoologists to the chief sub-divisions of the animal kingdom ; 

 its use has superseded that of the older term sub-kingdom. It is 

 a very usefal and correct term, because it reminds the student 

 that similarity of structure. is understood to be due to kinship 

 in race. 



Plastid (Gk. TrXctiTT-os, what has been formed or moulded or shaped). 

 — i-Ihe unit of form. . Both plants and animals are seen under 

 the microscope to consist of numberless little units of structure, 

 all much alike. Each is essentially composed of protoplasm, 

 with a thicker part near the middle, which is called the nucleus. 

 These units were named cells, because in the case of plant tissues 

 there is a hollow wall left where the units are cut through and 

 the juice runs out of them. But in animal " cells " this is not 

 the case, therefore the name " cell " is inapplicable. The name 

 plastid, which may be roughly rendered as " unit of form," has 

 been therefore proposed instead. 



Polyp (Gk. TToAiiTTos, or TroKiwovs, many-footed). — The name applied 

 to the units of colonial animals, from the fact that they usually 

 possess numerous tentacles, "arms" or "feet"; e.g. corals. 

 (In classical writers the name usually refers, however, to the 

 cuttle-fish, the tentacles of which may be called feet. 



Protoplasm (Gk. irpuTos, first, and irXda-na, form, i.e. formative 

 substance, from TrXacro-u, to mould). — Primitive basis of the body ; 

 the soft, jelly-like material of the cells of living bodies, some- 

 times spoken of.as the " physical basis of life." 



Proximal (Lat. proximus, nearest). — At the Jiear end, i.e. the fixed 

 end ; thus the shoulder is the proximal end of the arm. 



Pseudopodlum (Gk. tpevdris, false, iroiis, foot). — A "false foot," tem- 

 porarily, pushed out by the protoplasm of u, cell, and presently 

 retracted by the contraction of the protoplasm. 



Retrograde Development. — When the larval or early stages of an 

 animal have traces of higher organization than that of the adult 

 stage, it is concluded that the animal is descended from a higher 

 type, and has lost some of its organs, and become degraded. It 

 is then said to be a case of retrograde development. 



Sac (Lat. saccus, a bag). — Any bag-like structure. 



Sarcode (Gk. craVf, flesh). Flesh-like substance ; the protoplasm of 

 minute oi'gaiiisms. 



Segment (Lat. segmentum, a slice; from seco, to cut). — 1. One of the 

 successive similar joints of an articulated animal, such as u, 

 lobster. 2. One of the apparently similar parts into which the 

 developing egg-cell divides itself. 



Segmentation.— The process of division by which a developing egg- 



