The science of life : fully illustrated in tone and line and including many diagrams

BOOK 3

L.e.B.

Fig. 138. A Turbot recapitulates us symmetrical past. Above, a newly hatched larva, symmetrical, with organs incompletely developed, and a large yolk-sac. Next, about one-eighth of an inch long, symmetrical, independent, swimming upright. Next, one-fifth of an inch long ; the right eye is beginning to shift upwards. Last but one, four-fifths of an inch long ; the right eye has grown nearly round to the left side, the animal swims almost flat instead of upright. Below, fully transformed ; the fish spends most of its time bing on the bottom.

much more modern plants, like the sunflower, though cilia are no longer formed,

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THE SCIENCE OF LIFE

CHAPTER 3

the fertilizing male nuclei still assume corkscrew shapes in recapitulation of the coiled, swimming sperms of many lower plants.

Two final examples we may note, one from fish, the other from Crustacea. Everybody knows what soles and plaice and other flat-fish look like, and a great many people are aware of their peculiarity in having both their eyes on the one side of their twisted head and in lying flat, not on their belly, but on one side, the opposite side to the eyes. But not so many know that when they hatch out of the egg, flat-fish are symmetrical like other fish, swim about in the ordinary upright position, and have their two eyes on different sides of their head like any ordinary vertebrate. It is only after several weeks that a symmetrical growth distorts the head and eyes and the animal gradually settles down to its sideways existence. This is natural enough in the case of a fish which was once freeswimming like other fishes but which, generation by generation and age by age, has taken to life upon the bottom, but it is fantastic if we suppose that the plaice and soles were specially created as they are now. Why should they not spawn themselves in miniature ?

The most startling example has been reserved to the end. Portunion is the name given to a repulsive parasite found in the gill-cavities of crustaceans, where it devotes all its energies to sucking its host's blood and maturing its own reproductive cells. In the female sex especially this creature looks much less like a whole animal than like a detached piece of somebody else's internal anatomy. ‘There is nothing to show what sort of a creature it really is, from what kind of animal it has degenerated. But in its development it lets the cat out of the bag—or perhaps we should say it lets the seawoodlouse out of the bag ; for when young it is obviously a crustacean the details of whose anatomy place it at once in the order which contains the familiar slaters and woodlice. In Book 2, Chapter 2, we traced a similar story in the parasite Sacculina, which

is shown by its larva to be a curiously disguised and degenerate barnacle.