KEPOET ON THE CRINOIDEA. 275 



frequent absence of the ambulacral groove and its associated tentacular apparatus on 

 more or fewer of the arms. This is well shown both in Actinometra belli and in 

 Actinometra regalis (PL LXIV. fig. 2; PL LXVIII. fig. 1), and also in Actinometra 

 magnified (Part I., pi. lvi. fig. 7). The number of arms is very great in the last- 

 named species, and there are some without grooves on every ray, a condition which 

 also occurs in Actinometra nobilis. But as a general rule the ungrooved arms are 

 those which come off from the posterior part of the disk. Thus, for example, some or 

 all of the four posterior arms are very frequently ungrooved in the ten-armed types, 

 Actinometra Solaris and Actinometra pectinata ; while in other individuals of the 

 same species all the arms are provided with grooves, just as in Antedon. The same is 

 the case in the multibrachiate forms. I have seen one individual of Actinometra 

 parvicirra in which nineteen out of thirty-one arms were entirely devoid of an 

 ambulacral groove and tentacular apparatus, while in other specimens there is a groove 

 on every arm. 



It is then the potential, rather than the constant presence of ungrooved arms which 

 must be regarded as one of the distinguishing characters of Actinometra ; and the same 

 may be said of another peculiarity which is frequently associated with it, viz., the 

 difference in length of the anterior and posterior arms. This is less apparent in the 

 ten-armed than in the multibrachiate species, in which, however, it is sometimes very 

 distinct, e.g., Actinometra belli, Actinometra nobilis, and Actinometra regalis. The 

 anterior arms are much longer, taper more slowly, and contain far more joints than the 

 posterior arms, though these often have their genital glands better developed than the 

 anterior arms. In Actinometra simplex the tentaculiferous anterior arms have one 

 hundred joints, while there are only forty-five in the hinder arms, which have no 

 ambulacral groove nor tentacles. The two characters are not always associated, however, 

 for in the single specimen of Actinometra elongata all the arms are grooved and tenta- 

 culiferous, but the posterior ones have only fifty-five joints and reach but 4 - 5 cm. long, 

 while the anterior arms with one hundred and twenty joints reach 11 cm. 



This species is also remarkable for the presence in the later pinnules of the posterior 

 arms of those curious brown cellular bodies that I have supposed to be sense-organs 

 (PL LVII. fig. 4). I found them first in some specimens of Actinometra parvicirra 

 from the Philippines, 1 and have since detected them in an example of this species from 

 Banda, in Actinometra elongata from the same locality, and in Actinometra simplex 

 from the Admiralty Islands ; while they also occur in examples of Actinometra meri- 

 dionalis from two localities on the American coast. They are not always present in 

 either species and are generally confined to the pinnules of the hinder arms, sometimes 

 to one or two arms only ; but in one case I found them on all the arms except the 

 two immediately adjoining the mouth. I know not what these brown " ovoid bodies " 



1 Trans. Linn. Hoc. Lond. (Zool.), ser. 2, 1879, vol. ii. p. 40, pi. ii. fig. 6, o, b. 



