No. 19.] ECHINODERMS OF CONNECTICUT. 53 



yet well understood. A general account of the larval stages is 

 given in the earlier papers by Agassiz,* and by Mead.f 



The eggs fall to the bottom, and after fertilization the pro- 

 cess of cleavage takes place, as described for the egg of the sea- 

 urchin (page 100). In about twelve hours the substance of the 

 tgg has been subdivided into hundreds of cells which arrange 

 themselves to form the body of an embryo. The surface of the 

 body becomes covered with hair-like processes, termed cilia, 

 which are kept in constant vibration and enable the embryo to 

 leave the bottom and swim free in the water. 



The body of the embryo becomes further differentiated, and 

 a mouth, digestive system, and other organs are formed. The 

 embryo can now take into its body still smaller organisms than 

 itself for food, and under favorable conditions increases rapidly 

 in size. Meanwhile the rounded shape of the body has changed 

 into a structure with a number of projecting lobes covered with 

 very long cilia, and the embryo is now termed a bipinnaria. 

 In this stage the embryos are very sensitive to light and at- 

 mospheric conditions, and collect in vast swarms which at times 

 float near the surface and at other times sink deeper into the 

 water. They may be carried back and forth by the tides, winds, 

 and currents, and become widely separated from their birthplace. 

 They have a considerable power of locomotion, but do not swim 

 for a long time in any given direction. 



By further modifications the embryo develops into the strange- 

 looking, transparent creature shown in Figs. 4 and 5. In this 

 stage, known as the brachiolaria, the body is provided with 

 several pairs of long arms covered with vibratile cilia. As the 

 brachiolaria bears no resemblance whatever to the adult star- 

 fish, naturalists for a long time supposed it to be an entirely 

 different kind of animal. And yet from such a creature the 

 young starfish develops by a complicated metamorphosis. 



After swimming about in the water for some three or four 

 weeks, depending largely upon temperature and other external 

 conditions, the larva becomes full-grown, and the brachiolaria 



♦ On the Embryology of Echinoderms. Proceedings Am. Acad., 1863; and Mem. 

 Am. Acad., 1864. North American Starfishes. Mem. Museum of Comparative 

 Zoology, 1877. 



tThe Natural History of the Starfish. Bull. U. S. Fish Commission for 1899. 



