ME. P. H. CARPENTER ON THE GENUS ACTINOMETRA. 53 



Variety 3. 



Centrodorsal piece large and thick, with only 3 cirrhus-scars. 



Radidls. Second radials completely united all round. Centre of dorsal surface of the skeleton, from 

 the centrodorsal till near the end of the arms, marked by a faint white line with dark borders. 



Arms. 39. 



Syzygial interval. Usually 3, but varying from 1-7 segments; generally to <3. 



Comb. On second distiehal, palmar, and brachial pinnules, and occasionally also on those of the 

 3rd— 5th brachials, but on no others. 



Colour. Reddish brown. 



Locality. Bohol. 



Variety 4. 



Centrodorsal piece large and thick, with only 3 cirrhus-scars. 



Radials. Second radials closely united all round. Median white line on dorsal surface of skeleton 

 very marked. 



Arms. 33, all tentaculiferous, and tolerably uniform in length and in the character of their pinnules. 



Syzygial interval. Usually 3, but varying from 0-6 segments; generally to >3. 



Pinnules. Oral pinnules much stouter than in the type ; that of third brachial but little shorter than 

 that of second. Comb limited to these and to the distiehal and palmar pinnules, and the processes forming 

 it gradually come to rise from the ventral surfaces of the calcareous segments instead of from their outer 

 margins. 



Colour. Blackish brown. 



Locality. Ubay. 



IV. Tiie Skeleton. 

 (i.) The Skeleton generally with its Ligaments and Muscles. 



(§ 35) The general structure of the skeleton of Actinometra, and of the ligaments 

 and muscles which connect its component pieces, is precisely the same as in Antedon ; 

 and as this has been already described by Dr. Carpenter 1 , there is no need to repeat it 

 here : a few points, however, must be treated somewhat more in detail. The component 

 pieces of the skeleton of Actinometra, as of all the other Echinoderms, consist of a 

 calcareous reticulation formed by the calcification of an organic basis of a protoplasmic 

 nature, in which numerous nuclei and pigment-granules are imbedded. This " nuclear 

 tissue," as Simroth 2 has called it, is in the form of a network, around the meshes of 

 which the calcareous material is deposited. 



The character of the calcareous reticulation varies greatly in different parts of the 

 skeleton, being much closer at the synostoses and syzygia and at the articular surfaces 

 than in the interior of the segments ; and in correspondence with this greater compact- 

 ness of the calcareous tissue, the organic plexus which forms its basis becomes remark- 

 ably modified at these points, as will be seen further on. The various modes of union 

 of the different pieces of the Crinoid skeleton have been closely investigated by Miiller 

 and by Dr. Carpenter. The former 3 described the stein-segments of Pentacrinus as united 

 to one another in two different ways — (1) by the tendons which traverse the whole 



1 Phil. Trans, loc. cit. p. 702. : Op. cii.p. 433. 3 Bau des Pentaerinus, pp. 17-20. 



