CRINOIDEA. 279 



arms, and with the exceptions mentioned on p. 271 possess an 

 ambulacral groove and tentacles. This view of them is sug- 

 gested by their mode of origin in the growth of the arm, as 

 branches at the growing end.* The first indication of a pinnule 

 is the formation of a fork at the growing point of the arm ; one 

 of these branches grows faster than the other and forms the 

 continuation of the arm, while the other becomes a pinnule. In 

 Hyocrinus the proximal pinnules are almost as long as the arms 

 and the pinnule-bearing joints have the appearance of axillaries. 

 In the Cyathocrinidae there are no pinnules, but the arms are 

 much branched and what would be pinnules in other forms are 

 merely arm-branches. 



The second brachial is the first arm joint which bears a pinnule 

 (Thaumatocrinus, Eudiocrinus}. Pinnules are always absent 

 from axillary joints, from the hypozyal of every syzygy (see 

 p. 283), and from the lower of every pair of joints that are united 

 by ligamentous articulation. 



The pinnules contain the generative organs, but the so-called 

 oral pinnules of Antedon and its allies are sterile. In Meta- 

 crinus the ambulacral grooves of the proximal pinnules may 

 start directly from the margin of the mouth or from the portion 

 of the grooves on the calyx cover ; so that the pinnules appear 

 to be appendages of the calyx. 



The calyx-cover (tegmen calycis) in the simplest cases con- 

 sists only of the large triangular, interradially 

 placed oral plates (sometimes called deltoids), 

 which are arranged in such a manner as to form 

 a pyramid-like projection over the ventral side 

 of the calyx (Fig. 194). The outer sides of these 

 plates are in contact with the radials of the 

 calyx, and the posterior of them is, in Haplo- 

 crinus at any rate, perforated by the anus 

 (Fig. 194). This arrangement is found in the J e h r e fo ? a ? ng is ^ 

 Larviformia. In Holopus among living forms, Srffc* between 

 a very similar condition is found, there being a iadiai ght a nd Ster the 

 very few small plates between the large orals wScMs *he iSter- 

 and the edge of the calyx (the position of the ^\ al or radi ' 



* In Comatulids, however, the proximal pinnules are formed later than 

 the distal, and are not therefore formed at the growing point as arm- 

 branches. 



