DIVISION OF ANIMAL KINGDOM. 367 



form ot nucleated ceils, which manifest the common or- 

 ganic characters, but are without the true distinctive 

 superadditions of either kingdom. 



The animal kingdom is conveniently arranged under the 

 following primary groups, sub-kingdoms, or departments : 

 PROTOZOA, COSLENTERATA, ANNULOSA, MOLLUSCA, and 

 VERTEBRATA. These are again divided into classes, classes 

 into orders, orders into families, families into genera, and 

 genera into species. In the first-named department, the 

 Protozoa, is included a vast number of creatures of the 

 simplest organisation, classified as follows : 1. Gregarinidse, 

 2. Rhizopoda, 3. Polycystina, 4. Thalassicollid^e, 5. Spongidae, 

 6. Infusoria. It is not unusual to place Ehizopoda and 

 its typical form, Amoeba, in the front rank; and as the 

 presence of a mouth characterises the Infusoria, the re- 

 maining part of the Protozoa are frequently designated by 

 a collective name, AstomcOa. 



The first-named, G-regarinidce, form a group of the very 

 simplest structural character, and any one of them, setting 

 minor modifications aside, may be said to consist of a sac, 

 composed of a more or less structureless membrane, con- 

 taining a soft semi-fluid substance, in the midst of which 

 is a circular vacuole or vesicle, having in its centre a more 

 solid particle or nucleus. Professor Huxley appends to 

 this description the obvious, but highly important re- 

 flection, that its statements are all true concerning the ova 

 of any of the animals much higher in the scale. 1 The 

 Gregarinidce inhabit the bodies of other animals, and they 

 multiply by becoming encysted, and dividing into a multi- 

 tude of minute objects, called pseudo-navicellce, from their 

 resemblance in shape to the ship-like diatoms (naviculce). 

 When a young pseudo-nayicella escapes, it behaves some- 

 what like an amreba, and, if it chance to get swallowed 

 by an appropriate host, it grows into the parent form. 

 The whole life-history of these creatures is not known, as 

 they have not been traced into the exhibition of sexual 

 properties ; and it is therefore possible that their position 

 in the scale may not be exactly denned. 



In the course of the numerous investigations made of 

 the flesh of animals dying during the year 1866 from the 

 3attle-plague, it was noticed that large quantities of pecu- 

 liar bodies infested the muscular structures, more especially 



(1) Huxley's "Lectures on the Elements of Comparative Anatomy." 



