a certain shrimp (Figs. 4, 5, 6). Let us now compare these with the young stages of a number of shrimplike animals, viz. Sacculina, Lernæocera, Lepas, Cyclops, Limnetis (all drawn in Fig. 4), some of which lead a parasitic life. All start equally with the recapitulative phase known as the Nauplius; but whilst

the Nauplius of the free-living shrimp grows more and more elaborate, observe what happens to the parasites -they degenerate into comparatively simple bodies and this is true of their internal structure as well as of their external appearance. The most utterly reduced of these parasites is the curious Sacculina (Fig. 7) which infests Hermit-crabs, and is a mere sac filled with eggs, and absorbing nourishment from the juices of its host by root-like processes.

Lernæocera again, which in the adult condition is found attached to the gills of fishes, has lost the welldeveloped legs of its Nauplius childhood and become an elongated worm-like creature (Fig. 8), fitted only to suck in nourishment and carry eggs.

Amongst these Nauplii-all belonging to the great group Crustacea, which includes crabs and shrimpsis one which gives rise to an animal decidedly degenerate, but not precisely parasitic in its habits. This Nauplius is the young of the ship's Barnacle, a curious stalked body, enclosed in a shell of many pieces (Fig. 9). The egg of the Barnacle gives rise to an actively swimming Nauplius, the history of which is very astonishing. After swimming about for a time the Barnacle's Nauplius fixes its head

against a piece of wood, and takes to a perfectly fixed, immobile state of life (Fig. 10). The upper figures represent the Nauplius stage of animals closely resembling the Barnacle; the lower figures show the transformation of the Nauplius into the young Barnacle. Its organs of touch and of sight atrophy, its legs lose their locomotor function, and are simply used for bringing floating particles to the orifice of the stomach; so that an eminent naturalist has compared one of these animals to a man standing on his head and kicking his food into his mouth.

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.1

We have two of these represented in Figs. 11 and 12. The one (Fig. 11), as compared with a spider, is

1 I have, since the above was written, applied the principle of degeneration to the explanation of the living representatives of the group Arachnida, and shewn that the King Crab (Limulus) is the nearest representative of the ancestors of scorpions, spiders, and mites, and in fact must be classed with them. Professor Claus of Vienna has, some years later, adopted similar views. December 1889.

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 folliculorum) 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 de

generate, but which, I think, there is every reason to

FIG. 13.-Two adult Ascidians: to the left Phallusia-to the right Cynthia: the incurrent and excurrent orifices are seen as two prominences. Half the natural size.

FIG. 14.-A colony of compound Ascidians (Botryllus) growing on a piece of seaweed (Fucus). Each star corresponds to eight or more conjoined Ascidians. Natural size.

FIG. 15.-Anatomy of an Ascidian (Phallusia). At the top is the mouth, to the right the orifice of the cloaca. In the cloaca lies an egg, and above it the oblong nerve-ganglion. The perforated pharynx follows the mouth and leads to the bent intestine which is seen to open into the cloaca. The space around the curved intestine is the body-cavity; in it are seen oval bodies, the eggs, and quite at the lower end the curved heart. The root-like processes at the base serve to fix the Ascidian to stones, shells, or weed.

believe are the degenerate descendants of very much