Fig. 10.—Development of Cirrhipedes (Barnacle and Sea-acorn). After Huxley.
Were it not for the recapitulative phases in the development of the Barnacle, we may doubt whether naturalists would ever have guessed that it was a degenerate Crustacean. It was in fact for a long time regarded as quite remote from them, and placed among the snails and oysters; its true nature was only admitted when the young form was discovered.
Other parasitic organisms, which exhibit extreme degeneration as compared with their free-living relatives, might be cited and figured in profusion, did our limits permit. Very noteworthy are the degenerate Spiders—the mites, leading to still more degenerate forms, the Linguatulæ.
We have two of these represented in Figs. 11 and 12. The one (Fig. 11), as compared with a spider is seen still to possess the eight walking legs, small, it is true, whilst the palps and daggers of the spider have dwindled to a beak projecting from the front of the globular unjointed body. In the other the eight legs have become mere stumps, and the body is elongated like that of a worm.
Fig. 11.—Acarus equi. A degenerate Spider or mite parasitic on the skin of the horse.
Fig. 12.—Degenerate Spider (Demodex foliculorum) found in the skin of the human face.
The instances of degeneration which we have so far examined are due to parasitism, except in the example of the Barnacle, where we have an instance of degeneration due to sessile and immobile habit of life. We may now proceed to look at some sessile or immobile animals which are not usually regarded as degenerate, but which, I think, there is every reason to believe are the degenerate descendants of very much higher and more elaborate ancestors. These are certain marine animals, the Ascidians, or sea-squirts. These animals are found encrusting rocks, stones, and weeds on the sea bottom. Sometimes they are solitary (Fig. 13), but many of them produce buds, like plants, and so form compound masses or sheets of individuals all connected and continuous with one another, like the buds on a creeping plant (Fig. 14).
