94 STUDIES IN EVOLUTION 



contrary process ; namely, that of beginning with spines and 

 passing through stages corresponding to laminae, ridges, 

 tubercles, etc. The spine is the limit, and out of it no 

 further structure is formed. 



It is necessary to make some mention here of the movable 

 spines of Echinodermata, which appear to form an exception 

 to the foregoing statements. There seems to be no doubt 

 that the fixed and movable spines, the pedicellarise, the 

 paxillse, and the spheridia, are homologous structures, and 

 that all begin as spiniform skeletal outgrowths, which by 

 subsequent growth and modification produce the structures 

 mentioned (Agassiz ^). The echinoderm skeleton, including 

 spines, etc., is deposited in the midst of living tissue, and in 

 the case of the spines cannot be directly correlated with the 

 spines of other classes of organisms, which are either very 

 deficient in vitality or are dead structures as soon as com- 

 pleted. After the movable spines of echinoderms are fully 

 developed, the living portion is often confined to the base, 

 and the shaft becomes simply a dead structure upon which 

 encrusting organisms may find lodgment, a condition seldom 

 occurring in the living spines. These finished spines never 

 develop into anything else, and are the structures which 

 conform to the present discussion. The embryonic condition 

 of the spines and pedicellarise shows that they are really more 

 internal than external structures, and therefore remain under 

 the full control of the ordinary processes of growth, resorp- 

 tion, and modification by living tissues. Furthermore, the 

 movable spines are of such functional importance that no 

 close homologies can be made with ordinary spines found in 

 other classes of organisms. 



In tracing the ontogeny of a spinose form, it has been 

 found (pp. 18-22) that each species at the beginning was 

 plain and simple, and at some later period spines were grad- 

 ually developed according to a definite sequence of stages. 

 Usually after the maturity of the organism the spines reach 

 their greatest perfection, and in old age there is first an 



