162 LECTURE XIV. 



bifid. The back of the animal is covered, like the Argulus armigery 

 by an ample shield, terminating anteriorlj'- in two extended horns, 

 and posteriorly in a simple elongated spinous process (c). 



The discoveries of Mr. Thompson have been confirmed by Audouin, 

 Wagner, and Burmeister. The latter entomologist divides the de- 

 velopment of the Cirripedes into five stages. The first is that of the 

 ovum, the second of the locomotive embryo, the third when the 

 young attaches itself and becomes encased in a shell, in the fourth 

 stage it gradually assumes the character of the adult, and the fifth 

 stage is that of perfect development. 



The locomotive embryo is developed before the ovum quits the 

 parent : the shell, in the first stage of its growth, is coriaceous, and 

 formed of one piece, which is placed on the back. The organs by which 

 the young animal fixes itself are the long antennae, or setigerous legs, 

 situated near the mouth : in the Lepas anatifera the peduncle is formed 

 by a sac-shaped process of the mantle filled with yellowish matter. 



The general course of this metamorphosis, and the enjoyment of 

 locomotive and visual organs for a brief period, which are wholly 

 denied to the full-grown animal, characterize a condition which is 

 closely analogous to that of the young in the Epizoa, in the Trematoda, 

 and, with the exception of the visual organ, to the metamorphosis of 

 the sessile Polypes. 



LECTUEE Xiy. 



CRUSTACEA. 



In the part of the Animal Kingdom which we have hitherto surveyed, 

 we have seen the organs of the vegetative functions rapidly acquiring 

 high and complex conditions, and again sinking to a primitive and 

 simple type. The digestive canal may present an oesophagus, a giz- 

 zard, a glandular stomach, and an intestine in a polype, and then be 

 reduced to a radiated sac, with one aperture in the star-fish. The 

 development of the organs for the propagation of the species is sub- 

 ject to a like oscillation ; the dioecious condition is early acquired, and 

 all the accessories to the essential glands, even marsupial sacs ; but 

 the generative system again subsides to an androgynous combination 

 of simple ovaria and testes in the red-blooded worms. 



The organs of the animal functions manifest a steadier and more 



