MR, P. H. CAEPEXTER OX THE GENUS ACTIXOMETRA. 39 



fewer of tlie arms have uo tentacular apparatus ; but in the fourth variety (PI. I. fig. 10) 

 all the arms are of the usual character, with open ambulacral grooves fringed with 

 crescentic leaves and groups of tentacles. I have found the same variation to occur also 

 in AcL Solaris. In this case the number of arms is limited to ten, which may be all tcn- 

 taculiferous, or from one to four of the aboral arms may have no tentacular apparatus. 



{Note. February 1879.— No less than 23 out of 48 species of Acthionietm brought 

 home by the ' Challenger ' have more or fewer grooveless arms. I have cut sections of 

 these arms in two species, and have obtained the same results as with Act. polyniorpha 

 and Act. Solaris. The "ventral nerve" and ambulacral epithelium arc conspicuous by 

 their absence, while the axial cords in the skeleton give off branches freely in the centre 

 of each arm-joint, as I have ah-eady described for other species, both oi Aiitedou and of 

 Actinometm. Two points are noteworthy. In one species one of the posterior ambu- 

 lacra stops quite abruptly on the disk, and the two arms to which it would naturally 

 have gone, with its " nerve," tentacles, &c., receive no branches from any of the adjacent 

 grooves to supply the deficiency. Again, in one of the largest Comatiilce I have ever 

 seen (a 'Challenger' specimen from the Philippines) there are more, than 100 arms, 

 many of which are both grooveless and nerveless, as I have found from sections. But 

 these abnormal arms are not limited to the hinder part of the disk as is usually the case ; 

 for there are several on each radius.] 



The distribution of the non-tentaculiferous arms in Act. pohjmorpha varies, like their 

 number, to a very great extent. In any case they always occur on the odd posterior radius, 

 D (PI. I. fig. 8) ; when more are developed they may occur on the posterior divisons, 

 C. and El, of the two lateral radii, C, E ; and they may then be called postero-lateral 

 (PI. I. figs. 6, 12-14) ; and when the proportion of non-tentaculiferous to tentaculiferous 

 arms becomes very great, more or fewer of the antero-lateral arms, Ci, E,, belong to the 

 former class (PI. I. figs. 7, 9-11, 13), while in exceptional cases non-tentaculiferous arms 

 may occur even on the anterior radii (fig. 15, A). 



(§ 26) The condition of the ambulacral groove and of the tentacular apparatus is not 

 the only point in which the anterior or oral may dhfer from the posterior or aboral 

 arms. The former taper very slowly, contain far more segments, and are much longer 

 than the latter, while the form of their terminal portions and of the pinnules which 

 these bear is altogether diff'erent (PL I. figs. 4, 6). When viewed from the dorsal side 

 (PI. II. fig. 7) the basal portions of the two kinds of arms are precisely similar ; they 

 widen slightly between the first and second syzygia, i. e. from the third to the tenth 

 bi-achial, remaining unifoi-m till the third syzygium on the fourteenth brachial, after 

 which they begin to taper. Up to about the twenty-fifth or thirtieth segment the oral and 

 the aboral arms decrease in width at about the same rate ; but from this point onwards 

 there is a great diff^erence between them. The arms borne by the two anterior radii, 

 A and B, taper very slowly, the length of their segments increasing considerably, while 

 the breadth only diminishes very gradually ; at the same time the middle and terminal 

 pinnules, in which no genital glands are developed, become very long and filiform, and 

 remain so until the last few segments, when then- length suddenly diminishes very 

 considerably (PI. II. fig. 4). 



