THE STAR-FISHES. 163 
given on the Table (p. 160), furnish various proofs of the 
validity of the law of progress by their historical develop- 
ment and by the systematic development corresponding to it. 
As however these subordinate groups of Molluscs are in 
themselves of no further special interest, I must refer to the 
sketch of their pedigree on p. 161, and to the detailed 
pedigree of Molluscs which I have given in my General 
Morphology, and I shall now at once turn to the consider- 
ation of the tribe of Star-fishes, 
The Star-fishes (Echinoderma, or Estrelle) among which 
are the four classes of Sea-stars, Sea-lilies, Sea-urchins, and 
Sea-cucumbers are one of the most interesting divisions of 
the animal kingdom, and yet we know less about them 
than about any. They all live in the sea. Every one who 
has been at the sea shore must have seen at least two of 
their forms, the Sea-stars and the Sea-urchins. The tribe of 
Star-fishes must be considered as a completely independent 
tribe of the animal kingdom on account of its very peculiar 
organization, and must be carefully distinguished from the 
Animal-plants—Zoophytes, or Coelenterata, with which it is 
still frequently but erroneously classed under the name 
Radiata (as for example, by Agassiz, who even to this day 
defends this error of Cuvier’s, together with many others). 
All Echinoderma are characterized, and at the same time 
distinguished from all other animals, by a very remark- 
able apparatus for locomotion, which consists of a eompli- 
cated system of canals or tubes, filled with sea water from 
without. The sea water in these aqueducts is moved partly 
by the strokes of the cilia, or vibratile hairs lining their 
walls, and partly by the contractions of the muscular walls 
of the tubes themselves, which resemble india-rubber bags. 
