joints, and the Zoëa becomes thus modelled (Fig. 135) into the exact form of a Mysis or opossum-shrimp (Fig. 131). Finally, the single and median eye disappears, the outermost of the two end joints of each of the chest-limbs disappears, leaving these walking legs (seen so plainly in shrimp, prawn, crab, and lobster) of single conformation; gills are developed within the chest, sense-organs appear, and the full development of the prawn (Fig. 132) is then completed. Throughout these varied stages it is not difficult to trace a panoramic succes

sion of forms accurately reproducing the existing degrees and forms of the crustacean class. The early Nauplius (Fig. 133), the zoëa or water-flea stage (Fig. 134), the mysis-form (135), each produced in definite and advancing succession, present us with a perfect picture of the evolution of the prawn-race from lower crustacean life, and, presumably also, of the evolution of all other crustaceans belonging to the same rank and series in the class.

In summarising the results to which a study of the development of the echinoderms and crustaceans leads, there is to be recognised the operation of the principles already more than once insisted upon in the preceding pages, namely, that community of descent is provable by likeness in development, just as differences. or obliterations and alterations in development are explicable on the grounds of adaptation and change acting concurrently with the evolution and progress of the race. Only by taking into account these two principles, can the hard ways of development be understood. The present subject is one which may be regarded as lying thoroughly without the province and power of any explanation not founded upon evolution and upon the idea MYSIS-STAGE OF PENÆUS. that progressive change is part and parcel of the order of nature. Admitting that the only feasible explanation of these curious phases of development is to be found in such an idea of nature's constitution, it seems folly to deny that the general weight of evidence in favour of descent more than counterbalances any difficulties which may present themselves in connection with the exact determination of the lines along which that descent has travelled. That larval or young forms are themselves liable to modification from various circumstances must be admitted. This variation (to be hereafter studied in the insect-class) of the young form, which we regard as representing the primitive stock of the class, must unquestionably complicate the study of

FIG. 135.

evolution and add to the difficulties of constructing a perfect pedigree of the living world. The Pluteus larva of a sea-urchin and the Bipinnaria larva of a starfish, thus differ in respect that the former possesses a limy framework which is wanting in the latter. But such distinctions do not in the least degree militate against the primary fact underlying all such developments, namely, that the likenesses, not merely of young forms, but in adult structure, are explicable only on the theory of a common origin. Indeed, with the best of reason and logic, it may be argued that, as a condition of evolution, we postulate the occurrence of variations in the young stages as well as in the adult form-just as we should legitimately expect to find in living horses. the rudiments of those toes which the ancestors of the existing equine race possessed. Thus "direct" development, such as we have seen to occur in some starfishes and sea-cucumbers, whereby the young pass directly into the form of the adult, and wherein the changes of structure and appearance are suppressed, is a result of the adaptation of the larvæ to new ways of life. Rejecting this view, we should have to fall back upon the anomalous position of maintaining that there existed for one echinoderm a law of special creation, and for another a law of descent-a supposition which no logical mind will accept, and which the grander idea of the uniformity of nature at once dispels. As a final remark in connection with the sea-urchin class and its transformations, we may add that the changes in form are themselves progressive in nature. The five existing groups of this class (sea-urchins, starfishes, sand-stars, sea-lilies, and sea-cucumbers) are unquestionably modifications of a common plan of structure, and they originate from a larva which is wonderfully similar throughout, if we consider the diversities of adult structure which arise therefrom. Further, if this larva were to be arrested in its development and to represent a mature form in such an arrested stage, it would present a striking resemblance to some of the lower worms and their allies; this fact alone pointing to the probable beginnings of the sea-urchin class in a worm-stock. No less clearly do we see in the varying degrees of organisation exhibited by adult echinoderms, the same proof of progressive advance and modification of an originally primitive type. The forces and powers which, before our waking eyes to-day, evolve a sea-urchin from its egg and easel-like larva, or a starfish from its Bipinnaria, are, if we will only consider the wonderful nature of the transformations involved, engaged in as evident and intricate a work of evolution as those which have developed the varied twigs and branches of the Echinoderm tree in the æons of the past.

The foregoing conclusions find, perhaps, plainer illustration in the history of the Crustacean class, wherein exists a uniformity not so clearly traceable—although its original existence may not be doubted-in the early life of the echinoderms. The highest members of the crustacea are, as we have noted, the lobsters, crayfishes, shrimps, crabs, and

their allies. We have seen that in the crayfish a "Nauplius "-stage is represented; that in the lobster a Zoëa-phase is seen; that Mysis likewise exhibits a Nauplius, and then settles down as a peculiar form; that in the crab's early history, a still better marked Zoëa appears; and finally, that the shrimp Penæus actually passes through a Nauplius phase, a Zoëa or water-flea stage, a Mysis form, and finally assumes the likeness of the shrimp tribe. The history of Penæus, therefore, is practically an abridged treatise of the evolution of all higher crustacea: its development, to parody Pope's line, is "not one, but all Crustaceans' epitome." And as perfectly are the facts of lower crustacean life correlated with those of the higher development of the class. A water-flea, like Cyclops, as an adult, matures its development and ceases to progress at a stage corresponding to that at which Penæus has but attained its youth. The barnacles and sacculinas exhibit the influence of conditions of parasitism acting at a definite stage in the course of ordinary development, and producing the degraded and attached form of the adults. Mysis advances so far on the way towards the lobster and crayfish type, but stops short in its development at a point represented in lobster history, and beyond which the lobster itself passes as we have seen. Finally, beyond all such stages, and underlying all the variations and obscurities even of the higher and most modified life-histories, we see the Nauplius-form continually appearing as the starting-point of all crustacean history; or as that point, to use Fritz Müller's expression, which represents the "extreme outpost of the class, retiring furthest into the grey mist of primitive time." The Nauplius appears before us, then, as the founder of the crustacean race. The Zoëa is a modification and advance upon the Nauplius; and from this Zoëa (as poved by Penæus-development) were evolved the higher crustaceans at large. The lobsters and their allies (again appealing to Penæus) were evolved from the Zoëa-form through an intermediate stage represented to-day by the Mysis or opossumshrimp; whilst the short-tailed crabs, in all probability, arose directly from the zoea, without the intervention of a Mysis-stage, seeing that in their development they exhibit a distinct Zoëa-stage, and do not pass through a Mysis-stage like the lobsters and their long-tailed neighbours.

Diagrammatically expressed, we may see in the history of crustaceans that tree-like arrangement of their pedigree which best illustrates the deductions of evolution. The Nauplius exists at the root of the class. Developed in direct line, we find Penæus passing through the Zoëa and Mysis-stages. The lobster branch diverges after the Mysis-stage has been attained, and the crabs depart from the main stem before the latter phase. The crayfish, with its obliterated Nauplius-stage, may be presumed to have followed the course of development resembling that of the lobster; its history,

however, being singular in respect of the obliteration of the intermediate stages. The king-crabs have presumably originated in the common Nauplius-form, and have passed through the Trilobite-form, now extinct, to their present position at the extremity of an isolated branch of the crustacean tree; although, indeed, some naturalists hold that the king-crabs are more nearly allied to the spiders and scorpions, than to the Crustaceans. The barnacles, fish-lice, and water-fleas, obviously nearly allied, spring from a distinct Naupliusstem, but diverge through different ways and paths of life—the former to exist mostly as degraded parasites, and the latter to develop into Penæus (fig. 132),

active free-swimming forms. Thus becomes clear to us the meaning of those singular changes in animal forms which puzzled the older naturalists. To question the meaning which evolution attaches to these changes, is to leave them without explanation or meaning. Our knowledge of the full evolution of the Crustacea or any other animal group, as already remarked, may be, and often is, far from perfect. We are, it is true, still in the "grey mists" of many biological subjects, and the pedigree of animals, amongst others, is still enveloped in much obscurity; but, at the same time, we can detect breaking through the mist, gleams of knowledge-bright forerunners of that flood of light which the research of after-years will assuredly bring.

XI.

THE EVIDENCE FROM DEVELOPMENT (concluded).

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III. THE DEVELOPMENT OF MOLLUSCS, AMPHIBIANS, &c. THE attempt has been made in previous chapters to show that in the development of living beings there lies an enormous store and fund of evidence which goes either directly to support evolution as a rational theory of the universe, or which, at any rate, aids us in comprehending the causes which have, directly or indirectly, made the world of life the wondrous thing it is. The result of our inquiries has been to show that in the first beginnings of an animal's development, and in its earliest phases of progress, there is an amazing likeness to the early stages of every other animal's progress towards maturity. But even after these early similarities have appeared, there may be demonstrated in many groups a later likeness, which may often be traced beneath forms of the most diverse kind. The progress of the living being is unquestionably, as Von Baër aptly put it, one from the general to the special. Thus a sponge, a sea-squirt, and a man, may and do agree in the essential phases of their earliest development. But the special features of each group of sponges, seasquirts, and quadrupeds are soon respectively assumed, and, finally, there appear those more defined structures which mark the completion of development, and which land us within the class, order, or even species to which each belongs. Development may thus be compared to a journey in which all the travellers, or developing animals, start from a common point, and in which all pursue at first a common path that shortly, however, branches out into numerous diverging roads and routes, each leading to the goal or destination of the race. Community of origin is proved by two animals following the same beaten track for a longer or shorter distance; dissimilarity arising when their pathways diverge and the route divides.

A

FIG. 136.-MUSSEL.

Shell opened, showing ligaments, muscles (cc),

and foot().

a