ECHINODERMATA. 



105 



the water-vascular or ambulacral system appears to take no 

 part in locomotion, and the reproductive organs are lodged in 

 the lateral processes of the arms. The most remarkable point, 

 however, about the Comatula is the manner in which it de- 

 velops itself. When fully grown (Fig. 35, a) it presents no 

 small superficial resemblance to some of the Ophiuroidea. 

 When young (Fig. 35, b) the Comatula is so different in ap- 

 pearance from the adult, that it was originally described as a 

 distinct animal. It consists now of a little cup-shaped disk 

 with ten radiating arms above, produced by the splitting into 

 two of five primary rays, and furnished inferiorly with a little 

 flexible column or stalk composed of a number of calcareous 

 joints. By this jointed stem the body is at this period of 

 life fixed to sea-weeds or other submarine objects. When 

 sufficiently mature, however, the body drops off its stalk, and 

 then only requires to grow in size to become a fully-developed 

 Comatula. 



The stalked condition which we have just seen to consti- 

 tute a merely temporary stage in the life-history of the Coma- 

 tula is, on the other hand, the permanent state of parts in 

 almost all the " stone-lilies " and other fossil Crinoidea, and 

 in two or three living forms. Of these recent species, one of 

 the most remarkable is one which has been recently discovered 

 in the Atlantic and North Seas, and which has been described 

 under the name of Mhizocrinus lofotensis. This curious species 

 (Fig. 36) consists of a little thread-like, jointed stem support- 

 ing a calcareous cup, from which proceed 

 five branched and jointed arms ; and the 

 stalked condition is here permanently re- 

 tained during life. 



ORDERS V. AND VI. CTSTOIDEA AND 

 BLASTOIDEA. These orders merely require 

 to be mentioned here, as all the forms in- 

 cluded in them are extinct, and are unrep- 

 resented at the present day by living spe- 

 cies. In both, the body is enclosed in a 

 kind of box formed by jointed calcareous 

 plates (Fig. 37), and it was in most cases 

 permanently fixed to the sea-bottom by a 

 jointed stalk or column. The arms, which 

 form so conspicuous a feature in the true Crinoidea, were 

 either absent or very rudimentary. Both orders are most 

 closely allied to the Crinoidea, and they constitute probably 



FIG. 37. Cystoidea. 

 EcMnosphwrites. 



