142 . LOWER INVERTEBRATES. 



calyx is more or less covered over by the oral plates, usually five in number. 

 These are separated by narrow spaces that are continuous with grooves that run 

 along the upper surface of the arms and pinnules. The water system is upon the 

 usual echinoderm plan, that is, a ring around the mouth, and radial vessels running 

 along the arms. The ambulacral feet upon the oral surface of the calyx are connected 

 with the ring canal. The body cavity extends into the arms, which also contain the 

 greater part of the ovaries, as in the star-fishes. 



In Antedon and other free crinoids there is a cavity known as the chambered 

 organ, which has its walls and floor formed almost entirely by the centro-dorsal tubercle, 

 and therefore, by what was once a stem segment. This chambered organ is rudimen- 

 tary in the cystidean or unarmed phase of growth, but becomes developed as the coma- 

 tula acquires arms and cirri, and is connected with these parts by fibro-cellular cords 

 in the axis of the calcareous part of the arms and cirri. These cords are believed to 

 be nerves, and the chambered organ must therefore be regarded as the centre of the 

 nervous system. The ' ovoid gland,' which some consider to be a heart, is implanted 

 on one of the horizontal floors of the chambered organ. 



In Pentacrinus the chambered organ is a part of the central space enclosed within 

 the pentagon formed by the radials and basals, while in the Apiocrinidis {Rhizocrinus 

 and its relatives) an intermediate condition exists. The ovaries (in Antedon) dis- 

 charge their ova from openings on the arm pinnules. The eggs are fertilized while 

 attached to the exterior of the opening, undergo total segmentation, and after awhile 

 develop into oval embryos with a surface covered with cilia. When the embryo leaves 

 the egg, it is girded with four zones of cilia, bears a tuft of cilia at one end, has a 

 mouth (surrounded by large cilia), and an anal opening, and is free-swimming. In a 

 few hours or days, traces of the calcareous plates, destined to form the cup of the 

 future crinoid, begin to appear ; then the plates of the stalk develop, and lastly the 

 basal plate. As is the case with all echinoderms, there is little in common between 

 the larva and the young of the perfect form. The young crinoid is formed within the 

 larva, and the mouth and digestive cavity of the latter are not converted into those of 

 the former. Two or three days after the appearance of the plates, the larva begins to 

 change its form, the cystid-like young crinoid is seen embedded in the body of the larva, 

 and the latter sinks to the bottom and adheres to some object. The stem becomes more 

 elongate, while the part which will be the calyx still remains short and thick. The 

 broad end of this part becomes five-lobed, each lobe answering to an oral plate, and 

 these plates open like the petals of a flower, showing the oral aperture. Tentacles 

 then appear between the oral plates, eventually arranging themselves in groups of 

 three. Alternating with the basal and oral plates, the five radial plates now appear, 

 and the arms gi-ow out rapidly. The calyx also widens, so that the oral plates be- 

 come widely separated from the basal, which encircle the stem. The alimentary canal 

 of the young crinoid, which has before been a mere sac, now develops an intestine 

 which opens out on an interradius where an anal plate has now aj^peared. If the ani- 

 mal is a stalked crinoid, the principal further external alterations are the acquisition 

 upon the stem of whorls of cirri at intervals, the bifurcation of the arms, and the 

 development of pinnules upon them. 



In Antedon or Comatula, Actinometra, and Atelecrinus, forming the family Coma- 

 TULiD^, the young are stalked and attached, but the calyx, together with the upper- 

 most joints of the stem, bi-eaks off at a more advanced period of life, and the crinoid 

 swims off freely. Articulated to the lower or aboral face of the centro-dorsal tubercle 



