NO. 7 SEA-LILIES AND FEATHER-STARS CLARK 3I 



accomplished through the intermediary of insects of various types, 

 more rarely by small birds, which transport the pollen through the air, 

 and the plants have developed all sorts of artifices by which they make 

 their flowers attractive to these creatures. The result of this neces- 

 sity has been to localize in the flowers the chief differential characters 

 of the plants just as the same characters have in the crinoids and in 

 the other plant-like animals been chiefly segregated in the commonly 

 flower-like food-collecting mechanism. 



The polyps of the plant-like animals cover the maximum area with 

 their arms or tentacles in order to collect the maximum amount of 

 food, while the flowers cover the maximum area with their petals in 

 order to attain maximum visibility, with the common result of a 

 circular expanse of symmetrically arranged parts in both cases. To 

 increase their efficiency by mass effect the polyps of plant-like animals 

 are often spiked, sometimes spirally arranged on the axis, and 

 occasionally grouped in umbels or in imperfect racemes like flowers, 

 while to counteract unusual external stresses of waves or wind both 

 flowers {Raoulia, etc.) and polyps (brain-corals, etc.) are sometimes 

 in the same way gathered together in great more or less globular and 

 highly resistant masses. 



A very large proportion of the conspicuous flowers are pentapartite, 

 with five sepals and five petals alternating with them, and commonly 

 bracts beneath the sepals ; crinoids are also pentapartite, with five 

 basals and five arm-bearing radials alternating with them, and com- 

 monly infrabasals beneath the basals. Some flowers are tetrapartite, 

 like the crucifers; some crinoids are the same, like Tetracrinus. 

 Many flowers are hexapartite, as are also some crinoids, like Hexa- 

 criniis. The reason for the most common occurrence of five in both 

 cases is probably that in the pentapartite division there lies the maxi- 

 mum strength. 



The basals of the crinoids normally enclose the visceral mass much 

 as in many flowers the sepals enclose the ovary, and sometimes 

 {Isocrinus, etc.) they imbricate over the bases of the radials as the 

 sepals imbricate over the bases of the petals. 



In many crinoids the arm bases are firmly united by interbrachial 

 plates or so closely pressed against each other that they may almost 

 be said to possess a gamopetalous corolla. 



In a few fossil crinoids {Petalocrinus and Crotalocrinus) all the 

 arms borne by each radial are united into a single, broad, flat plate 

 which may be highly flexible, and the crowns of these crinoids 

 resemble flowers to a most astonishing degree. 



