36 INVERTEBRATE ANIMALS. 



larger than the one before it, and the coils of the spiral all 

 lying in one plane, then we get such a shell as Discorbina 

 (Pig. 4, c). This is one of the commonest forms of shell among 

 the Foraminifera, and it is often called the " nautiloid " shell, 

 from the close resemblance which it bears in shape to the 

 well-known shell of the pearly Nautilus. It was, in fact, this 

 external similarity which induced the older naturalists to 

 place the Forammifera among the MoUusca in the neighbor- 

 hood of the cuttle-fishes. There are numerous other types of 

 shell, all of which can be referred to the manner in which 

 gemmation is carried on by the primordial segment ; but the 

 two forms above mentioned may be taken as sufficient ex- 

 amples. It may be mentioned, however, that there are forms 

 in which the new segments are added in a very irregular man- 

 ner, and the resulting colony has no very definite shape, as in 

 Glohigerina (Fig. 4,/"). 



Affinities of the Foraminifera. — In spite of their beautiful, and in 

 many cases complex, shells, the anatomical structure of the Foraminifera is so 

 simple that it may fairly be questioned whether in a systematic arrangement 

 they should not be placed at the bottom of the whole sub-kingdom Protozoa. 

 Perhaps the nearest relatives of the Foraminifera are the Polycyaiina, a 

 group of organisms which we have yet to consider. These differ from the 

 Foraminifera in little or nothing, except that the shell is composed of flint. 

 The Forarninifera are also clearly related to those forms of the Amcelea 

 which possess shells, such as Difflugia. The sarcodebody of Diffiugia, 

 however, contains a nucleus and a contractile vesicle, and the pseudopodia 

 are thick and blunt, so that the differences are suflSciently weighty. There 

 are also very interesting points of relationship between the Foraminifei-a 

 and the sponges, which cannot be touched upon here. A few words, how- 

 ever, may be said on the physiological deductions which may be drawn from 

 the study of the Foraminifera. Regarded from a physiological point of 

 view, the structural simplicity of the Foraminifera renders them all the 

 more wonderful. We have in them the great equation of life presented to 

 us in perhaps its simplest form. They are composed of an organic substance, 

 but cannot be said to possess " organization," being " structureless, and 

 without permanent distinction or separation of parts." * Nevertheless they 

 perform all the physiological functions ; they assimilate food — they live, 

 grow, and maintain their integrity in the face of the destructive forces con- 

 stantly at work upon them — they reproduce their like — and they have cer- 

 tain relations with the external world, being at any rate capable of indepen- 

 dent locomotion. All these vital actions they effect without possessing a 

 single organ permanently set apart for the performance of any one of them. 

 Lastly, they have the power of building up an outer envelope or shell, which 

 is always beautiful, and is often of the most complex character, and con- 

 structed upon a regular mathematical plan. The Foraminifei-a, then, of all 

 known animals, offer the most convincing illustration of two laws : firstly, 

 that there is something in the action and nature of vital forces altogether 

 distinct from any thing hitherto observed in the physical forces ; and sec- 



♦ Huxley. 



