DEVELOPMENT OF THE OYSTEE-CEAB. 745 



propagate rapidly, and that the branches of the curious tree-lik^ colonies would also increase in 

 numbers. The individuals of the colonies multiply in about three ways : first, by branching ; 

 secondly, by splitting lengthwise; thirdly, certain much enlarged and overfed zooids divide cross- 

 wise. By the two last modes one-half of the product is often set free, the free animalcules 

 so originated being known as "swarmers." These cast-off or free zooids which drop from the 

 colonies are no doubt carried along by the vortex created by the cilia of the gill and palps, and 

 hurled into the mouth and swallowed as part of the daily allowance of the food of the Oyster. 

 We may therefore regard Pinnotheres, in such instances, as a veritable nursery, upon the body and 

 legs of which animalcules are continually propagated and set free as part of the food supply of 

 the Oyster, acting as host to the crab. I do not suppose, however, that such a condition will 

 always be found to obtain, and it must also be remembered that myriads of Zoiithamnium colonies 

 were dredged up attached to the fronds of the handsome Grinnelia, a red alga commonly found 

 in certain parts of Chesapeake Bay. Where this plant grows in abundance on the bottom I have 

 estimated that one might find upwards of a hundred animalcules attached to a square inch of 

 frond surface, which would indicate an animalcular population of upwards of four millions of 

 individuals to the square rod, a number as great as that of the human inhabitants of the city of 

 London, 



Development op the Oystee-crab. — The Oyster-crab undergoes a development and 

 metamorphosis similar to that of our edible crab, Oallinectes, but the body in the Zosea stage 

 is blotched with dark, branched pigment cells. The eyes also are vastly more developed than 

 in the adult, where they are partly suppressed from disuse. There is no dorsal spine, nor are 

 the antennary and rostral appendages so well developed as in the Zoaea of Callinectes. After the 

 young are hatched they probably leave the abdominal covering of the parent, swim out of the 

 Oyster for a season, and, if female, seek a permanent abode in some Oyster near by, behaving 

 somewhat like the species described by Semper as inhabiting the water-lungs of certain holothu- 

 rians. After undergoing furthtsr development, the young Pinnotheres reaches the megalops stage 

 of its development, when it is probable that the choice' of its home takes place.. After it has 

 entered the mantle cavity of its host as a diminutive larva, and has grown to be adult, when 

 it measures a half inch or more in diameter, it is probably ever after a prisoner within the 

 shell of its moUuscan protector. It undergoes a retrogressive metamorphosis as it grows adult, 

 its eyes become relatively less conspicuous than in youth, and it never has a thick, hard shell 

 like its allies which live in the open water, but the external skeleton remains almost entirely sott 

 and chitinous, or in the state in which we commonly find the outer covering of an edible crab 

 which has just molted. This arises apparently from the conditions by which the animal is sur- 

 rounded; the protection afforded it by its host does away with the need of a thick, hard covering 

 such as we find Inclosing the bodies of its free-swimming allies. Unlike the latter, too, the limbs 

 of the Oyster-crab are to some extent degenerate and weakened; its chelae or claws are feeble, 

 and, when removed from its home, seems a very sluggish, helpless sort of creature, without a 

 particle of the pugnacity of its allies, and if placed on its back will sometimes remain in that 

 position helplessly beating the air with its weak limbs. This is a remarkable instance, which also 

 serves very admirably to illustrate the principle of degeneration in organic evolution, so ably 

 dealt with by Prof E. Eay Laukester. 



The Oyster itself is also an example of the eff'ect of disuse in producing retrograde develop- 

 ment, and even shows signs of gradual adaptation when removed from one locality to another. 

 Unlike most other bivalves, the Oyster has, no soft muscular foot which it may protrude outward 

 from between the edges of its valves. No visible rudiment of such a prominence can be found 



