DISTRIBUTION OF ORGANISMS. 151 



varieties flourishing in the early seas, but which are now 

 nearly extinct. The creature consisted of a stomach and 

 arms, surrounded by long tentacles or arms, placed upon the 

 top of a stalk fixed to the sea-bottom, the whole being com- 

 posed of numberless minute calcareous plates, connected by 

 gelatinous substance. In more advanced forms of the same 

 order, (as the comatula and the extinct marsupite), the body 

 and arms desert the stalk, and betake themselves to a free- 

 swimming life ; but, as has been elsewhere mentioned, the 

 young comatula lives for a time as an encrinus ; that is, upon 

 a stalk. Seeing that the same animal, in an earlier embryotic 

 stage, represents a polypidom, we conclude that in the poly- 

 piaria is the origin of the echinodermatous line : it is first the 

 polypidom, then the encrinus, then the free-swimming coma- 

 tula, or feather star, the last being one of the most graceful 

 animals in existence. In the higher genera of the latter 

 family, the tentacles are shortened and reduced in number. In 

 the Ophiurce, there are only five long and simple rays pro- 

 jecting from the central body. Afterwards, in the Asteriada, 

 or true star- fishes, the central part dilates step by step, until 

 it fills up the interstices between the rays, and the form be- 

 comes a pentagonal disk. From this there is a clear passage 

 to the Echinus or sea urchin, which is merely a spheroidal 

 animal in a calcareous case, through which numberless spines 

 or tentacles project, for locomotion and the collection of food. 

 This form again becomes elongated into the cylindrical soft- 

 bodied Holothuria, with a circle of tentacles at the oral ex- 

 tremity ; thence the transition is easy to the genus Fistu- 

 laridcs, animals externally worm-like, and possessing the 

 rudiment of a heart, with red blood in the arteries, so that, in 

 this last echinoderm, we may be said to have come nearly, if 

 not fully abreast with, the annelides, and to be approximating 

 to some of the humbler fishes. ( 72 ) The reader cannot fail to 

 have been struck by the great number of forms passed through 

 in this line, in comparison with any other, before leaving the 

 radiate sub-kingdom ; but, in reality, the echinodermata, 

 though of radiated form, are much superior to the rest of that 

 division in their organization, which is, if not complicated in 



