282 PHYLUM ECHINODERMATA. 



era. The ambulacra! grooves however have covering plates or, as in 

 some Comatulids, covering folds without plates. The anus is at the end 

 of a papilla in the posterior interradius, and its walls resemble in structure 

 the interambulacral area from which it arises. 



The covering plates of the ambulacral grooves are either attached to 

 the brachials or to a special set of lateral or side plates. 



The stem is composed of a number of ossicles united by 

 close sutures or by articulation. It is traversed by an axial 

 canal (see p. 288), and it may bear at intervals whorls of jointed 

 cirri, which contain a prolongation of the axial canal. The 

 cirri of the lowest pieces are in some forms root-like in appear- 

 ance and ramify in the muddy or sandy bottom on which the 

 animals live (Rhizocrinus) ; in this case the normal cirri may 

 be absent. In other cases the lowest ossicle is attached to the 

 substratum by a kind of cement (Pentacrinus). In growth the 

 addition of new pieces is confined to the upper end of the stem, 

 where they arise by intercalation between existing pieces and 

 (except in Flexibilia and some Articulata) between the stem 

 and the cup. In Flexibilia, etc., the top segment of the stem 

 is often fused with the infrabasals.. 



In Uintacrinus, Marsupites, Thaumatocrinus and Holopus 

 the stem is absent. In the Comatulidae it is present in the 

 young, but in later life the animal breaks away from it, retaining 

 only the top joint, on which several whorls of cirri are formed. 

 This top piece (Fig. 189, Cd) which remains attached to the calyx 

 in Comatulids may be formed of two or more joints fused, as 

 is suggested by the numerous whorls of cirri on it ; it is called the 

 centrodorsal piece of the calyx and fuses with the infra-basals. 

 It is uncertain whether Thaumatocrinus has a stem in the young 

 state ; probably it has. Whether Marsupites and Uintacrinus, 

 which were also without stems, were fixed or not cannot be 

 certainly determined. Holopus is attached by the broad base 

 of its calyx, but it is without a stem. 



The connexions between the skeletal pieces of Crinoids are of various 

 kinds. In studying them it must be remembered that the plates are laid 

 down as calcareous films in the connective tissue of the body, and that 

 these, as they increase into plates, remain connected by the uncalcified 

 connective tissue. When this tissue is well marked, the joint is said to 

 be a loose suture ; if it is contractile in function (muscular) we have a 

 muscular articulation ; such joints permit of movement of the connected 

 plates on one another. When the plates are closely applied together 

 and the intervening connective tissue is sparse, we have a close suture. 



