THE STAE-FISHES. I 63 



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 Estrellae) 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 peciiliar 

 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 compli- 

 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. 



