350 C. K Beeoher — Origin and Significance of Spines. 



of various kinds, as legs, branches, leaves, etc., have by atrophy 

 been reduced to spines. In each case, the parts in their devel- 

 opment pass through the various intermediate stages, and 

 clearly show that the spine is a result and not a mean. More- 

 over, none of these structures or organs is developed through 

 the 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 farther 

 structure is formed. 



It is necessary to make some mention here of the movable 

 spines of Echinoderms, which appear to form an exception to 

 the foregoing statements. There seems to be no doubt that 

 the fixed and movable spines, the pedicillariae, 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 1 ). 

 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 organ- 

 isms, which are either very deficient in vitality or are dead 

 structures as soon as completed. 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 lodge- 

 ment, 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 pedicillarise shows that 

 they are really more internal than external structures, and 

 therefore remain under the full control of the ordinary pro- 

 cesses of growth, resorption, and modification by living tissues. 

 Furthermore, the movable spines are of such functional im- 

 portance 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. 14-17) that each species at the beginning was plain and 

 simple, and at some later period, spines were gradually devel- 

 oped 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 over-production or 

 extravagant differentiation followed by a decline of spinous 

 growth, and ending in extreme senility with their total absence. 



There are abundant reasons for believing. that the radicles of 

 groups are undifferentiated and inornate, and whenever a class 

 has had a long existence, it has been by the continuance of such 

 radical types or by the development of secondary or tertiary 



