30 EVOLUTION IN THE PAST 



of the covering disc were in a small minority in the previous 

 Period (Ichthyocrinida). During the Silurian such forms were 

 more in evidence. Among crinoids with covered mouths 

 some highly speculative developments for dealing with the 

 force of currents were taking place. In certain forms the 

 arms were joined together by means of numerous branches 

 resulting in a sort of network enclosure (Crotalocrinus). In 

 a few other forms each branching arm was compacted 

 into a fan-shaped petal (Petalocrinus). These experiments 

 were not very successful, for wicker-crowned and petalled 

 crinoids did not appear in any subsequent Period. Other 

 forms had come out as sanitary reformers. The sanitary 

 arrangements of many crinoids were doubtless far from 

 perfect, as the waste products in passing away were liable to 

 mix with the food supplies. To ensure against this, some 

 forms had evolved long tubes calculated to carry the waste 

 clear of the arms (Mastigocrinus, Botryocrinus}. 



Various modifications had been undergone in other cases, 

 resulting in a freer play of the " arms," greater flexibility 

 of the stem, increased strength of the cup or body-covering, 

 and the out-growth of small clinging tentacles as means of 

 support. Indeed, crinoids seem to have attained their 

 greatest development in this Period ; but they all differed as 

 yet, in several structural details, from crinoids now living. 



BLASTOIDS Some related animals known as Blastoids were now on 

 the scene. So far as is known they were confined at this 

 time to North American waters. The most remarkable feature 

 about these " bud-like " creatures was the presence beneath 

 the body-covering of certain tubes, known as hydrospires. 

 Currents of water, it is supposed, passed through the tubes, 

 and respiration was thereby carried on. This was, un- 

 doubtedly, a less simple respiratory system than that of 

 breathing by pores, as was the general rule with crinoids ; 

 and it may have yielded a greater thrill of life. Some of the 

 animals (Troostocrinus) had the system more fully developed 

 than others (Cadaster}. And various experiments were, no 

 doubt, being made, for the creatures were as yet but tyros 

 in the maze of competitive life. 



STARFISHES Old-fashioned Cambrian forms of starfishes with ill-defined 



