[447] INVERTEBRATE ANIMALS OF VINEYARD SOUND, ETC. 153 
Among the Echinoderms there are no species that swim at the surface 
when adult, but most of them produce eggs which hatch into very re- 
markable larvie, 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-fish 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 larvie of Asterias vulgaris (A. pallida AG.) and A. arenicola (A. beryl- 
inus AG.) 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 Dré- 
bachiensis) in the same way. The Cribrella saguinolenta, (p. 407,) like 
several other star-fishes, does not have free swimming larvie, 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 different 
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 Amphipholis 
elegans (p. 418) found in this region, but others have free-swimming 
larve, and pass through a metamorphosis similar to that of Asterias, 
though the larve 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 Sertulariaand 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 species of Obelia, swim free in the em- 
bryonic state, and then develop into attached hydroids, which by bud- 
ding may produce large branching colonies of similar hydroids, but 
ultimately they produce another kind of buds, which are developed 
within capsules or gonothece. These soon become elegant, little, circu- 
lar, and disk-shaped jelly-fishes, which are then discharged and swim 
free in the water; 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, complicated 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 
