ANIMAL KINGDOM. 129 



ture, and arms proceeding from the margin of the disk; also a stem, 

 consisting of calcareous disks, by which, when alive, they are attached 

 to the sea-bottom or some support, so that they stand in the water and 

 spread their rays, like flowers, the mouth being at the centre of the 

 flower. One of the Crinoids is represented in Fig, 145, and another 

 in Fig. 148, p. 118, the upper part of the figure in each showing the 

 rays closed up, and the lower part the stem. The rays open out, when 

 alive, and then the animal has its flower-like aspect. The little pieces 

 that make up the stem, looking like button-moulds, are either circular, 

 as in Fig. 145 a, or five sided, as in Figs. 148 a, b, c, d. Under the 

 Crinidea falls the Comatula family, the species of which are free when 

 adult, but have slender arms proceeding from the back surface for 

 attachment. 



(2.) The Blastoidea or Pentremitids. — Having a symmetrical ovoidal 

 body, with five petal-like ambulacra meeting at the summit, without 

 proper arms, and attached by a stem like that of the Crinids. 



(3.) The Cystidea (from the Greek for a bladder), Fig. 146. — Ar- 

 rangement of the plates not regularly radiate. Arms, when present, 

 proceeding from the centre of the summit instead of the margin of a 

 disk ; in some, only two arms ; in others, replaced by radiating ambu- 

 lacral channels, which are sometimes fringed with pinnules. 



In ancient Crinids, the arms are not generally free down to the 

 base, but there is a union of their lower part, either directly or by 

 means of intermediate plates, into a cup-shaped body or calyx (as in 

 Fig. 145, and also Figs. 577, 578, under the Carboniferous age, p. 298). 



In Fig. 149, the plates of one of these cups, in the species Batocrinus longirostris M. 

 are spread out, the bottom plates of the cup being at the centre. The plates, it is seen, 

 are in five radiating series, corresponding to the five rays or arms of the Crinid, and 

 between are intermediate pieces. The three plates numbered 1 are called the basal, as 

 the stem is articulated to the piece composed of them ; 3, 3, 3 are the radial ; 4, 4, 

 supra-radial ; 5, brachial, situated at the base of the arms; 7 are intermediate plates, 

 called inter-radial; 8, another intermediate, the inter-supraradial. Sometimes, in 

 other Crinids, there is another series of plates, at the junction of the plates 1 and 3, 

 called sub-radial. Finally, the anal opening of a Crinid is situated toward one side of 

 the disk, it being lateral, as in the Echinoid in Fig. 147; an# the intermediate group 

 plates numbered 10 are called the anal. 



In the Cystids, the aperture is generally lateral and remote from the top, as in Fig. 

 146, while the arms come out often from the very centre. The Cystids are also peculiar 

 in what are called pectinated rhombs (see Fig. 146); that is, rhombic areas crossed by 

 fine bars and openings: the use of them is uncertain, — though they are probably con- 

 nected with an aquiferous system and respiration. The Cystids are the most anomalous 

 of Radiates. 



2. Acalephs. — The free jelly-like Acalephs have very rarely left 



any traces in the strata. But, besides these, many kinds pass, in their 



development, through a polyp-like state, and, as the common Hydra 



of fresh waters is included among them, the species are called 



9 



