54 EVOLUTION IN THE PAST 



Blastoids during this Period reached their climax, and 

 some very graceful forms appeared (Orophocrinus). Their 

 culmination, however, was not marked by any wonderful 

 display ; and they descended from their humble zenith with 

 some rapidity. By the close of the Period they seem one 

 and all to have collapsed ; and their annals, therefore, were 

 comparatively short. Possibly the somewhat complicated 

 breathing apparatus, which they had developed, proved un- 

 workable owing to changes in surrounding conditions. 

 STARFISHES Very little evidence has come down of Carboniferous star- 

 AND BRITTLE- fishes and brittle-stars ; but as their Devonian ancestors 

 STARS were not far short in development of modern forms, their 



evolutions can only have been of a trifling character. 

 EA-URCHINS Rigid-tested sea-urchins of Silurian origin were still in 

 some force (Palceechinus). At this time, however, they were 

 being superseded by some closely related forms in body- 

 coverings with the plates constituted in more modern manner 

 (Melonites). Urchins in flexible tests were still numerous, 

 but flexibility had been considerably reduced in some cases 

 (Arch&ocidaris}. Rigidity, it is clear, had long been the 

 pass- word through the ranks. So far as is known the plate- 

 rows on all tests, flexible and inflexible, continued excessive 

 in number compared with what ultimately became the rule. 

 SEA- Holothurians or sea-cucumbers, in skins embedded with 



CUCUMBERS minute wheels, anchors, and other limy products, were 

 certainly now in being (Archistrum). Their precise line of 

 descent is obscure, but there is no doubt that they and sea- 

 urchins and all other of prickly group had descended from a 

 remote common ancestry. Their nearest relations in Car- 

 boniferous waters were probably the fast-vanishing cystids. 

 LAMPSHELLS The decline of lampshells does not seem to have been 

 arrested ; but these persistent animals made a good display 

 in many parts of the world. Some forms, moreover, of 

 record size now made their appearance. The most successful 

 at this time were those clinging to resting-places in shells 

 provided with long spines (Productidce). Clingers of this kind 

 had appeared as early as the Silurian, but neither in that 

 nor in the succeeding Period were they much favoured by 

 fortune. Their prosperity, therefore, was long in coming ; 



