Echinodenna. 



Ul 



is almost spherical^ somewhat longer than broad, with a saddle- 

 shaped depression on the ventral surface. The edge of the saddle 

 is a thickened ridge covered with cilia, which enables the animal 

 ro swim. The mouth is situated anteriorly in the depression, the 

 anus is posterior to the hinder edge of the cUiated ridge. In front, 

 the ridge bounds a projecting lappet {b Fig. >(' A), which in s<jme 

 cases is only connected by a narrow bridge with the rest of the 

 convex surface (Fig. SO £, Holothurian), or again, may even be 

 completely cut ofE from it, forming a special island in the concave 

 region, surrounded by a small ciliated ridge iFig. fn C. Starti>h'i. 

 In older larvae, the ciliated rim is more or less lobed, usually forming 

 long processes or arms, which are then frequently supported by 

 dehcate internal calcareous rods, as in the Ophiurids and Sea-urchins. 

 After some time, a portion of the larva undergoes complicated 

 modifications to form the body of the adult, whilst the rest gradually 

 shrivels up. The final product of the metamorphosis is a small 

 animal possessing the chief features of the adult, although differing 

 from it in many respects, e.g., in the small number of tube-feet. 

 The adtdt is thus produced by a remodelling of the larval 

 body, large portions degenerating whilst others become further 

 developed and modified. In some Echiuoderms, especially in th'jse 



Kg. SI. 

 J. SlflDer. 



A B C D 



Larrse of: A Starfisii. B Ophinrid, C Sea-urchin. D Holotlmiiaji. — After 



which are viviparous, a metamorphosis does not occur; or it is not 

 obvious; or ii is modified in various ways. 



Asexual reproduction occui-s in only a few forms; ste 

 Asteroidea and Holothuroidea. All the Echinoderma are marine, 

 hving at various depths ; they crawl about at the bottom, or are 

 sessile : only exceptionally are they capable of swimming. The 

 group is represented in the oldest fossiliferous sn-ata, and on account 

 of its abundance, and the frequent presence of a well-developed 

 dermal skeleton, fossils are very numerous. 



Class 1. Crinoidea • Sea-Lilies). 



The Crinoids are piimarily distinguished fi- jm other Echinoderms, 

 in that, either in the adult condition, or at least in the earlv stat'es 

 following free larval existence, they are attached to the sea-bottom. 



