II] THALAMOPHORA 25 



parts in a definite order, but upon this principle of the repetition 

 of similar parts the bodies of the most complicated animals are 

 built up. It is no doubt fundamentally the same thing as repro- 

 duction, only that the various units which are produced, instead of 

 separating from each other and leading separate existences, remain 

 connected, and, as we say, are co-ordinated to form an individual 

 of a more complex kind. In Polystomella the various parts are 

 called chambers; a name which properly belongs, and was first 

 applied to, the segments of the shell enclosing them. It is worthy 

 of note that it is only the protoplasmic body of Polystomella which 

 shows this composition out of definite units arranged in a definite 

 order ; there may be one large nucleus or a considerable number of 

 smaller nuclei, but they are not arranged in correspondence with 

 the chambers. 



The Thalamophora, which, like Polystomella, have compound 

 shells, are very numerous and include an immense variety of forms, 

 the variety being brought about by differences in the number of 

 chambers and the way they are arranged in series. They almost all 

 live in the sea; some, like the two we have described, creep about 

 amongst the sand and debris at the bottom of pools or other places 

 where the water is quiet; many others float at the surface of the 

 ocean, the protoplasm which clothes the outside of the shell having 

 numerous vacuoles filled with fluid probably less dense than the 

 sea-water, and thus serving as floats. In such inconceivable myriads 

 do these floating Thalamophora exist, that their empty shells form 

 thick banks of impalpable white chalky mud at the bottom of the 

 ocean, and the familiar white chalk of our English cliffs and hills is 

 largely made up of the shells of Thalamophora. 



When such a form as Polystomella is about to reproduce by 

 fission the protoplasm emerges from the old shell and divides 

 repeatedly into a number of pieces each as large as the central 

 chamber of an ordinary individual. These then secrete round 

 themselves shells and begin to bud off new chambers and gradually 

 acquire the size and shape of adults. When on the contrary 

 Polystomella forms gametes, the nucleus seems to break up into a 

 chromidium like that of Arcella, which is dispersed throughout 

 the protoplasm, and the protoplasm whilst still within the shell 

 divides into a large number of small rounded pieces, each of which 

 contains one of the minute fragments of the nucleus. These rounded 

 bodies are gametes ; they acquire hair-like projections called flagella 

 which can be moved to and fro and by means of which they can 



