CRINOIDS OR SEA-LILIES. 663 



each arm to the mouth, which is placed at or near the centre of the upper 

 surface of the body. The stream of water, containing numerous animalculae 

 and food particles, passes through the mouth into the gut, which is coiled round 

 the inside of the body, and passes out again at the anal opening. This latter 

 is often raised on a small eminence or papilla (Fig. 15, As). In some forms, 

 especially in some extinct genera, this papilla was extended into a long tube, 

 so that the excrement was carried far away from the food-grooves of the arms 

 (Fig. 13). Ranged along the sides of the food-grooves are the tube-feet, which, 

 however, are not provided with suckers, but probably serve as respiratory 

 processes. The small size of the body is perhaps to be accounted for by the 

 fact that the generative glands do not develop within the body, but extend 

 along the arms underneath the food-groove into the pinnules. Here the 

 generative products become ripe and swell up the pinnules, from which they 

 ultimately burst. The calcareous case that encloses the organs of the body is 

 known as the calyx; it may be regarded as composed of a cup supported like 

 a wine-glass on a stem, from the edges of which cup the arms originate ; 

 while it is covered by a lid in which are the openings for the mouth and 

 anus. The lid is pierced by pores which put the water-vascular system in 

 connection with the exterior. These may either be dispersed over the 

 surface of the lid, or, as in other echinoderms, collected in one plate the 

 madreporite. This plate, when it exists, lies between the mouth -and the 

 anus. In the crinoids now living the cup is composed of only two or three 

 circlets of plates, with no more plates than five in each circlet, and in many 

 cases the two lower circlets are hardly to be distinguished. In some ancient 

 crinoids, however, the organs of the body take up much more room, 

 and to accommodate them the cup had an increase in size. This was 

 effected, not by the plates becoming larger, but by the lid being, as 

 it were, raised up the arms, so that the lower plates of the arms were 

 included in the walls of the cup. At the same time, further plates were 

 intercalated between the arms and their branches, binding them all into 

 a plated integument, which looked like a tesselated pavement. 



The stem is perhaps the most interesting part of the crinoid anatomy. All 

 echinoderms, and many other animals, including, oddly enough, some which 

 are generally reckoned by zoologists among the ancestors of the Vertebrata, 

 have in their youngest stages a small process or lobe extending in front 

 of the mouth, by which lobe they are accustomed to attach themselves as 

 though by a sucker. In many cases the attachment 

 lasts but a short period ; but in the crinoids, at all 

 events, it persists for some time, and the lobe 

 becomes extended, while rings of calcareous sub- 

 stances are deposited in its integument. The 

 increase of these rings in number and in size 

 gradually produces a stem or column formed of 

 superposed ossicles, perforated by a canal which is 

 an extension of the original body-cavity. In the 

 crinoids this canal contains extensions of blood- 

 vessels and also of a sheath of nervous substance 

 surrounding the blood-vessels. These extensions 

 pass from a central five-chambered organ placed 

 at the bottom of the cup, just at its junction 



with the stem. From this strands of the same nervous substance also 

 pass in criss-cross fashion through all the plates of the cup, and finally join 



