454 THE CAT. [chap. xiii. 



of tliese so widely different animals. Amongst the lowest animals 

 of all — the Ilypozoa — we find conditions which remind us of still 

 earlier stages of the cat's existence, and also of fragmentary portions 

 of its adult frame. Amongst those lowly parasites the Gregoruiida, 

 are animals which consist only of a spheroidal particle of protoplasm 

 enclosed in a cell-wall and containing a nucleus and nucleolus — quite 

 comparable therefore with the cat's ovum. Amongst other lowly 

 creatures called Flagellata, some consist of a small spheroidal particle 

 of protoplasm, with a filamentary prolongation, or tail, by the 

 lashings of which the little creature propels itself along. Such 

 organisms are evidently comparable with the cat's spermatozoa. 

 Yet other lowly organisms amongst the Bhizopoda consist of par- 

 ticles of protoplasm which slowly change their shape in an altogether 

 irregular manner, and which quite resemble the white corpuscles of 

 the cat's blood. Such animals are called Amoebce, and it was with 

 reference to them that the motion of these blood-corpuscles was 

 spoken of as " amoeboid," or " amoobiform." 



But these creatures have a further interest for us. Each Amoeba 

 feeds both by imbibing nutritious fluid and by actually engulphing 

 within its substance undissolved nutritious particles, and this process 

 is comparable with what must take place in the nutrition of the 

 protoplasmic particles which form the ultimate parenchyma of the 

 cat's body. Again, each Amoeba effects a gaseous interchange with 

 its surrounding medium, gives out carbonic acid and takes in oxygen, 

 and this process is directly comparable with that intimate and 

 internal process of respiration before described as taking place 

 throughout the particles of the cat's parenchyma. Finally, many 

 of these lowly animals have the poAver of secreting Avithin their 

 unicellular bodies and extruding from them various formations of 

 different kinds, and this process is also compai-able with the like 

 actions of the epithelial cells which line the ultimate gland-tubules 

 of the cat, and also with that universal process^ of minute and 

 ultimate excretion which is carried on by the particles of its ultimate 

 body-parenchyma. 



But the whole of the sub-kingdom Ilypozoa has also an interesting 

 resemblance to the embryonic condition of such an animal as the 

 cat at a period anterior to its differentiation into distinct tissues. 

 For all the Ilypozoa consist either of single cells or of more or less 

 simple aggregation of cells, and in no hypozoon are these collected 

 and differentiated into tissues — not even into an cpiblast and hypo- 

 blast — conditions which appear for the first time in the sub-kingdom, 

 S2)ongida. 



§ 9. Inasmuch, then, as the cat is a hackboned animal, it may 

 be said to differ from the whole of the Ina'ektebrata in the follow- 

 ing points : — 



(1) Its body consists, in the adult as well as in the young con- 

 dition, of two unequal antcro-posteriorly elongated cylinders 

 — one placed dorsally, the other ventrally. 



