NO. 7 SEA-LILIES AND FEATHER-STARS CLARK II 



which may be opened outward or closed together over the mouth. 

 The plates of the radial series appear first as mimite spicules in the 

 angles between the basals and orals, one in each of these angles, or 

 sometimes two in the right posterior angle. These grow very rapidly 

 and two additional plates appear beyond them, the outer giving rise 

 to two plates each of which forms the base of a rapidly growing arm. 

 The additional plate in the right posterior area, known as the radianal, 

 is always present, often appearing before any of the radials, but 

 sometimes, as in Antedon, not being formed until a relatively ad- 

 vanced stage has been reached; it grows very slowly and by the 

 growth of the radial just to the right of it it is shoved gradually 

 upward and to the left, so that when the radials have come into lateral 

 contact and have united into a ring it occupies a position at the edge 

 of the disc where it is soon resorbed. Though small and soon 

 disappearing in the young comatulid this is a very important plate 

 in many fossil types. The arms at first are very different from the 

 arms of the adults ; they are composed of a series of exactly similar 

 brachials without pinnules and with all the articulations between them 

 the same, without muscles, and crossing the arm at right angles. 

 Pinnules first appear at the tip of the growing arm after from nine 

 to fourteen brachials have been formed, these being followed by the 

 pinnule on the second brachial and considerably later by the pinnules 

 on the intervening brachials (fig. 55). The first cirri appear on the 

 now enlarged topmost columnal just before, simultaneously with, or 

 just after, the first formation of the pinnules. Just before the 

 appearance of the pinnules and cirri and before the disappearance 

 of the radianal the larval comatulid is a remarkably perfect replica 

 of a fairly typical representative of the Flexibilia Impinnata, but 

 after the appearance of the pinnules and the cirri the crown under- 

 goes a most extraordinary transformation and rapidly assumes all 

 the characters of the adult comatulid, at various periods between five 

 or six months and two and one-half years breaking away from the 

 column and becoming a free-living feather-star. 



The fully grown pentacrinoid young of different species show great 

 variation, and range between 15 mm. and 65 mm. in total length; in 

 some the column is very short and relatively stout, not more than one- 

 third of the total length, and composed of as few as 10 segments, 

 while in others it is very slender and much elongated, reaching four- 

 fifths of the total length and being composed of more than 65 

 segments. 



In the comatulids and in the pentacrinites (possibly excepting 

 Metacrimis) with more than 10 arms the young always have 10 arms 



