ETC. 447 



Amoug the Echinoclerms there are uo species that swim at the surface 

 when adult, but most of them produce e^^gs which hatch into very re- 

 markable larva?, entirely unlike their parents in form and structure, and 

 these swim free in the water, often for a considerable period, by means 

 of vibrating cilia. 



The young star-lish or sea-urchin develops gradually within the body 

 of the larva, on the water-tubes, and as it grows larger it gradually ab- 

 sorbs the substance of the larva into its own body. The development of 

 the '[nYvx of Asterias vulgaris (A. pallida Ag.) and A. areni^ola {A. heryl- 

 imts AGt.) has been described by Mr. A. Agassiz, from the time pre- 

 vious to hatching from the eggs till they become young star fishes, with 

 the essential characters of the adults. He has also described the young 

 of the common green sea-urchin (under the name of Toxopneustes Dr'6- 

 hachiensis) in the same way. The Crihrella saguinolenta, (p. 407,) like 

 several other star-fishes, does not have free swimming larvae, but retains 

 and protects the eggs by holding them by means of the suckers around 

 the mouth, curving the body around them at the same time. In, this 

 position the eggs hatch and pass through a metamorphosis diflereut 

 from that of Asterias, though somewhat analogous to it. The develop- 

 ment of this species was described by Professor M. Sars many years ago. 

 Some of the Ophiurans are viviparous, among them the AmpMpliolis 

 eJegans (p. 418) found in this region, but others have free-swimming 

 larvce, and pass through a metamorphosis similar to that of Asterias, 

 though the larvae are quite different. Some of the Holothurians are also 

 viviparous, while others have free-swimming larvae, but the young of 

 most of the species of this region are still unknown. 



The Acalephs all swim free in one stage or another of their existence. 

 Some of the Hydroids, like Sertularia and allied genera, are only free-swim- 

 mers while in the early embryonic stages, when they are covered by vi- 

 brating cilia ) but they soon become fixed and ever after remain attached 

 in one place. Others, like the sx)ecies of Ohelia, swim free in the em- 

 bryonic state, and then develop into attached hydroids, which by bud- 

 ding may i)roduce large branching colonies of similar hj^droids, but 

 ultimately they produce another kind of buds, which are developed 

 within capsules or gonothecae. These soon become elegant, little, circu- 

 lar, and disk-shaped jelly-fishes, which are then discharged and swim 

 free in the w^ater ; they soon grow larger, acquire more tentacles, and 

 ovaries or spermaries develop along the radiating tubes, the eggs are 

 formed, discharged, and fertilized, and each egg may develop into a 

 ciliated embryo, which in its turn may become attached and start a new 

 hydroid colony. Thus among these animals we find an alternation of 

 generations, comi)licated by different modes of budding. 



In the case of the large red jelly-fish, Cyanea arctica, and the com- 

 mon whitish jelly-fish, Aurelia flavidula, (Plate XXXVI, fig. 271,) the 

 history is somewhat different. These jelly-fishes produce immense num- 

 bers of minute eggs, which are discharged into the water and develop 



