CRINOIDEA. 123 



certain period of its existence, however, the calyx drops off its 

 column, and becomes a locomotive animal. It now has a 

 near resemblance to one of the Brittle-stars (Ophiuroidea) ; 

 but is distinguished, not only by its developmental history, 

 but by the possession of lateral pinnae to the arms, and in 

 having the reproductive organs situated external to the body 

 proper. In the Feather-stars ( Comatula or Antedori) the dorsal 

 surface of the disc, at the point where the column was origi- 

 nally inserted, carries a series of jointed filaments or ".cirri," 

 by which the animal can moor itself to any foreign object. 

 These may be regarded as homologous with the " side-arms" 

 of the column of certain Crinoids. When the animal is thus 

 temporarily moored by its dorsal cirri, it is placed in the 

 ordinary position held by the Crinoids namely, with the 

 mouth and ventral surface of the disc looking upwards. When 

 creeping about, on the other hand, by means of the long and 

 flexible arms, the animal occupies the position held by the 

 Star-fishes and Ophiuroids namely, with the mouth and ven- 

 tral surface of the disc directed downwards, or towards the 

 ground. 



Having now given a general account of the structure of the 

 Crinoids, it remains to consider some of their parts in greater 

 detail. In the first place, as regards the " column," we find 

 that the stem of attachment is composed of a great number of 

 separate plates or " articulations " placed one above the other, 

 and so jointed to one another that whilst the amount of move- 

 ment between any two pieces must be very limited, the entire 

 column acquires more or less flexibility. The column is per- 

 forated by a round or five-sided canal which pierces every 

 joint, and runs along the entire length of the stem. In the 

 Palaeozoic Crinoids, with few exceptions, the column was 

 round ; but in Platycrinus it is oval or elliptical (fig. 73). In 

 the genera Pentacrinus (fig. 78) and Extracrinus the column is 

 pentagonal in outline ; but much less markedly so in the 

 former than in the latter genus. The joints articulate with 

 one another by surfaces or facets which are differently marked 

 in different cases. In the Palaeozoic forms, as in Platycrinus 

 (fig. 73), the articulating facets are marked by more or less 

 numerous striae which radiate from near the centre of the joint. 

 In most of the Mesozoic genera, on the other hand, as in 

 Pentacrinus (fig. 78), the articulating facets are united by 

 crenated ridges arranged in a pentapetalous figure. In many 

 cases, as in Extracrinus and Pentacrinus, the column is fur- 

 nished with more or less numerous "auxiliary" arms, or "side- 

 arms," the function of which is not altogether clear. 



