Young and adult specimens of Sacculina, to illustrate the Degeneration or Retrograde Metamorphosis which the parasite undergoes in the course of its development. A.-The Nauplius stage, in which the young Sacculina hatches. The three pairs of appendages correspond to the antennules, antennæ, and mandibles of a Crab or Crayfish. The black spot between the antennules is the eye, and the small patch immediately behind the eye and between the two hinder pairs of appendages is the ovary, which is already present at this very early stage. Unlike the Nauplius stage of other groups of Crustacea the Sacculina Nauplius has no mouth or alimentary canal. × 90. B.-The Cypris or pupa stage. It is at this stage that the young Sacculina, hitherto a free-swimming animal, attaches itself to a Crab and becomes parasitic. The pupa is characterised by the bivalved carapace; the stout antennules by which it fixes itself to the Crab; and the six pairs of locomotor appendages. The black spot is the Nauplius eye, and the mass immediately below it is the ovary. × 90. C.-The Sacculina three days after fixing itself to the Crab. The six pairs of swimming legs have rotted away and fallen off; the bivalved carapace is being detached, and is carrying with it the eye and certain débris from the body. The sole parts remaining, out of which the adult Succulina will be formed, are the antennules, now modified into a tube, which is represented projecting through a piece of the skin of the Crab; and the head, which forms a bottle-shaped mass attached to the tube, and containing the ovary. × 90. D.-Adult Sacculina attached to the ventral surface of the tail of a Crab (Portunus). The Sacculina is the large dark-coloured bag in the lower part of the figure; it is attached to the Crab by a short fleshy stalk, not seen in the figure, which, penetrating the skin spreads out into a tuft of branching roots in the Crab's body. X essential features. Embryology tells us that this means that the three animals must really be members of the same group, and allied to one another; and thereby gives us a clue to the real affinities of barnacles and Sacculina that we could hardly get in any other way. (Figs. 9, 10, 11.) In parasitic animals generally the shape and structure are liable to be so profoundly modified, in consequence of the special conditions of parasitic existence, that, but for the aid afforded by development, we should often be absolutely unable to determine to which division of the animal kingdom they really belong. This leads us to our second question : If Embryology gives the clue to the relationship of animals, may it not do more and reveal their ancestry ? THE RECAPITULATION THEORY. A further explanation is afforded us by what is known as the Recapitulation Theory, which states that not merely have existing animals descended from ancestors which are often unlike them, but that each animal bears the mark of its own ancestry and reveals its parentage in its own development. Evolution tells us that each animal has had a pedigree in the past; Embryology reveals to us this ancestry, because every animal has an inherited tendency during its own development to repeat its own ancestral history; or, to put it in other words, to climb up its own genealogical tree. A good example of recapitulation is afforded by flat fish such as the sole, flounder, turbot, and plaice, which are distinguished not merely by the remarkable flattening of the body from side to side, but by the further facts-(1) that the two sides, right and left, of the fish are never coloured alike, one being nearly white, and the other dark-coloured ; and (2) that the two eyes, instead of being situated, as in other animals, one on each side of the head, are both on the same side-i.e., the darkly-coloured one. On watching these flat fish in an aquarium, we note that they habitually lie on the bottom on the paler coloured side, and we are at once led to associate the remarkable condition of the eyes with this habit; for it is clear that when so resting, if the eyes were placed in the usual positions, one at each side of the head, the eye on the paler surface-i.e., the surface on which the fish lies-would not only be perfectly useless, but would be liable to injury from contact with the sea-bottom. On turning to the development of the flat fish we find this supposition confirmed. A sole on hatching, and for some time afterwards, has its eyes one on each side of the head, just like any ordinary fish; furthermore, it swims, like other fish, with the body vertical, and has its two sides coloured alike. It is only after it has attained some size that it gradually adopts the habits of the adult, and takes to resting on its left side on the sea-bottom and swimming with the body horizontal instead of vertical. At the same time, the right side of the body gradually becomes coloured differently to the left, and in such a way as to resemble the sea-bottom closely, and so enable the 1 fish to escape the notice of its enemies; and the left eye, no longer of use in its original position, is Young and adult specimens of the Flounder (Pleuronectes flesus), to illustrate the shifting of the eye from one side of the head to the other during the growth of the fish. A.-A young Flounder six days after hatching. The head is symmetrical, and the eyes one on each side. ×4 B.-A young Flounder, probably about a month old. The fish is gradually acquiring the characteristic shape of the adult, and the head is becoming twisted, the eye of the right side being displaced slightly downwards, and the eye of the left side coming into view over the top of the skull. ×4 C. An adult Flounder showing the characteristic shape of the flat-fish, and the complete migration of the left eye over to the right side of the head. gradually displaced upwards on to the top of the head, and then shifts over to the right side; the change in position of the eye being accompanied by very considerable twisting and distortion of the skull. (Fig. 12.) Inasmuch as flat fish in all other respects agree with more ordinarily-constituted fish, and as their special peculiarities-i.e., the lateral compression of the body, the difference of colouring on the two sides, and the singular position of the eyes-may all be readily and naturally explained by their habits, which again would be clearly advantageous to the fish in aiding them to escape from enemies, it becomes in the highest degree probable that flat fish are descended from normally-formed fish, which first acquired the habit of lying on one side for the sake of protection, and then underwent structural changes in consequence of this habit. If this be correct, then the developmental history of the flat fish becomes intelligible by assuming that each individual has an inherited tendency to repeat in its own development the history of the species; every flat fish during its own growth passing through the same series of changes by which we have supposed the whole race of flat fish to have acquired their special peculiarities. The case with regard to the sole is really a very strong one: for the only alternative view is that flat fish are not descended from normallyshaped fish, but have sprung into existence independently; and not only is this view absolutely contradicted by what we know of other animals, but it would render the development of the flat fish an incomprehensible mystery. The one view gives a |