February 1, 1899.] 



KNOWLEDGE 



31 



packing. One can only feel that this ia not an original 

 design, but an adaptation gradually brought about. So 

 anxious have the creatures been to have all the precious 

 items of their apparatus shielded within the valves of their 

 shelly portmanteau that they have wedged them together, 

 without much minding the natural order so long as their 

 efficient working was not interfered with. Even that point 

 has perhaps been a little sacrificed, seeing that some of 

 these essentially aquatic animals are without the essentially 

 aquatic accomplishment of swimming. This is ignominious, 

 but it is safe. They can walk, they can burrow, or they 

 can cling. Thus they can secure food and a contemplative 

 life without natatory gadding among the ferocious occu- 

 pants of the watery empyrean. 



There are four sections of the Ostracoda, one of which, 

 imder the name of Podoci'>pa, comprises fresh-water species 

 as well as marine, while only marine species are known 

 in the other three. In all of them the appendages, though 

 they can in general be identified in their serial order, differ 

 often very strikingly in their forms. This is scarcely to be 

 wondered at, since some of the species come from depths 

 of the tropical ocean, others from little grimy puddles on 

 land ; some have shells of polished smoothness, while 

 others show the most ornate rugosities ; some are less 

 than a millimetre long, others stretch their vast length to 

 seven or eight millimetres ; and one almost incredible mon- 

 ster from the Pacific is said to measure more than an inch. 

 The differences in the organs are not without a philo- 

 sophic as well as, doubtless, a functional value. Observe, 

 for example, the 

 mandible of -il/c^'rt/o- 

 cypris princeps 

 Sars, a princely 

 fresh-water ostra- 

 code from South 

 Africa. Its basal 

 joint forms a strong 

 transverse trunk 

 with a dentate 

 biting edge, such 

 as is so commonly 

 seen throughout 

 the Malacostraca, and from this trunk rises a jointed palp. 

 It has been already explained that the joints of a mandibular 

 palp must really correspond to the joints of a leg, in spite 

 of the utterly unleglike appearance of the ordinary 

 mandible. Turn now to the mandible of an Asterope. It 

 will be seen to be completely leg-like. Its basal joint is 

 not transverse but longitiidinal, end to end with the joints 

 which follow it, while the biting process, though present, 

 _ is quite unobtrusive. This compari- 



son is like the writing on a sign-post, 

 only, instead of directing us how to 

 reach a town or a village, it directs 

 us how to reach a conclusion. It 

 points towards this deduction — that 

 the strangely diversified forms of 

 mandible throughout the Crustacea 

 may have originated from a simple 

 string of similar joints. The form 

 of the mandible in ^ki/alm't/prif for- 

 cibly intimates that the Ostracuda 

 are not only connected with other 

 groups of Entomostraca but with the 

 Malacostraca as well. A genea- 

 logical unity of all the Crustacea thus springs into 

 view, and this can only be grounded on the theory of 

 evolution. Fifty years ago, in public estimation, that 

 theory was nowhere, and now it is everywhere. When it 



Mandible of Megalocypris pHncep.i . 

 Sars. 



Fi'om 



Mandible of Aitircrpe, 



sp. 



Brushing feet of Aiferope, 

 sp., slightly separated. 



first came into the open sunlight' it greatly affected not 

 only the thoughts but the lives of men. It made some 

 become naturalists who would otherwise never have given 

 more than a passing glance to natural history. Now, 

 indeed, it is so much taken for granted that men can hold 

 it, as they hold other great doctrines, with no grounds for 

 conviction and in placid companionship with a thousand in- 

 consistent views. But that is not the fault of the Ostracoda. 

 To return to the genus AsU'rope, in which the mandible 

 is like a leg, we find that here the penultimate pair of 

 appendages, which according to custom ought to be a pair 

 of legs, or at least leg-like, is more like a pair of flattened 

 out dust-pans. Its principal 

 or its most obvious use is not 

 wholly out of harmony with 

 the appearance. The strongly 

 spined terminal part of the 

 animal, known as the caudal 

 fork, in repose is folded 

 within the valves. In the 

 alternate unbending and re- 

 bending of this spring tail the 

 animal finds its most powerful source of locomotion. But 

 every alternate stroke protrudes the finely serrate spines 

 into the muddy or gritty surroundings of the outside 

 world, and some of the particles are carried back into the 

 scrupulously neat but limited accommodation within the 

 valves. There the legs, which have been modified into a 

 sort of combination of dust-pan and brush, receive between 

 their plates and hairs the bespattered spines of the tail, 

 and cleanse them. But here the old difficulty arises. 

 Quis custodiet ipsos oistodes ? Who is to brush the brushes ? 

 This question is answered by nature in a manner truly 

 remarkable. The next pair of limbs are converted into 

 what servants call "a general" and 

 what their employers call " a maid of 

 all work." In the family Cypridinida, 

 to which the genus Astt'rope belongs, 

 this leg " represents a long, vermiform, 

 annulate, extremely movable appen- 

 dage." It is employed to sweep the 

 outside of the valves, thus passing into 

 the water for its own purification, and 

 within them it brushes the brushes, it 

 plies between the laminar branchiffi or 

 breathing organs, and above all it 

 attends to the nursery, by working about among the eggs in 

 the brood-pouch, making things wholesome for the hopes 

 of the race, the coming generation of the Ostracoda, On 

 these and a hundred other matters of interest concerning 

 this group the student will find copious instruction in the 

 works of Sars, and Glaus, and Brady, and Norman, and 

 Robertson, and many others, to whom their writings will 

 introduce him, nor will he go far in his researches without 

 discovering that the fine monograph by G. W. Miiller is 

 indispensable to satisfactory progress. 



Annular cleansing 

 iootoi AstJrope, sp. 



SECRETS OF THE EARTH'S CRUST. 



By Grenville A. J. Cole, m.r.i.a., f.g.s., Professor of 

 Geology in the Royal College of Science for Ireland. 



INTRODUCTION. 



IN this seriea of papers the writer proposes to touch on 

 some of the questions that are prominent in the 

 minds of geologists at the present day. In so doing, 

 it must become clear in each case that the position 

 now reached is the result of a long series of observa- 

 tions, extending back throughout the century. There is 



