32 LOWER INVERTEBRATES. 



possess, besides tlie contractile vesicle and nucleus, the ability to protrude the body 



substance in the form of pseudopodia, by means of which they progress and take food; 



but they have in addition long vibratile flagella which place them in 



the higher group. 3Iastigamoeba simplex (Fig. 25) serves well to 



illustrate this small group. A similar form has been often taken by 



the writer from the soft mud and debris at the bottom of quiet water ; 



\j/^^ its movements are comparatively active, and its very long lash is 



1 /r\ < -i-^ thrust forward, beating the water with its rapidly vibrating extremity. 



Perhaps the most remarkable species of the Rhizo-flagellata is Podo- 



stonui filigerum of Claparede and Lachmann. It is very changeable 



in shape, and from the extremities of the pseudopodal protuberences 



tlagella may be produced or withdrawn at will. When these are 



not apparent, the animal closely resembles Amoeba radiosa, — indeed, 



Fig. 25— Uastiqam- I^iitschli has recently attempted to show that it is the same species; 



larged no times" ^"^' °" *^^® Other hand, it has been pointed out tliat the feet of Amoeba 



radiosa, however attenuate, are never thrown into spirals, nor vibrate, 



as in Podost07na. It sliould be looked for in infusions of hay. 



Sub-Okdee III. — Radio-flagellata. 



The Radio-flagellata, which follow very naturally the forms last mentioned, are 

 mostly marine. They may be compared to the Radiolaria, which they resemble very 

 closely in their ray-like pseudopodia, but, in addition, they are provided with one or 

 more lashes. Again, some of them are naked, while others are provided with silicious 

 cases or loricie. It should be remembered that these genera with tests are included by 

 Haeckel in the Radiolaria. Is the possession of flagella a sufficient distinction for thus 

 removing these to the Flagellate Infusoria ? Without considering the intermediate char- 

 acter oi Actinomonas and Actinolo2:>/ms jxduncnlatus, it would seem not; but these 

 forms bridge the chasm as well as that between the Rhizopoda and Rhizo-flagellata is 

 bridged. 



A characteristic example of the sub-order is Actinomonas mirabilis. Its body is 

 globular, supported on a thread-like stalk several times longer than the diameter of its 

 body ; from every part of the periphery radiate sarcode rays in search of food ; at the 

 top extends a long flagellum which, by its motion, causes water-currents to pass over the 

 rays. Food particles are taken, indifferently, at any part of the surface. 



Stjb-Order IV. — Pantostomata. 



We have now to consider true infusorian types where the injestive area is dif- 

 fuse, as in the preceding sub-orders, but which lack the pseudopodal appendages which 

 allied those groups so closely to the Rhizopoda. This extensive sub-order includes 

 eighteen families, divided into three groups ; viz. Monomastiga with one flagellum, 

 Dimastiga -with two, and Polymastiga with three or more. 



Every one who has used the microscope with any considerable magnifying power in 

 the examination of infusions, or the water of ponds, has doubtless seen minute globose 

 or elongate plastic bodies moving about by means of a single long thread placed at one 

 end of the body. Tliese forms belong to the genus Monas. As now limited, the 

 fanuly Monadid^, includes only the naked free-swimming species with one flagellum. 



