ECHINODERMATA. 



median line, and two smaller ones (6, i,) 

 placed laterally. On the sides of the ray the 

 calcareous substance is disposed, as it were, in 

 ribs (c, c, Jig. 9) ; these rise from the floor at 

 first nearly parallel with each other, and are con- 

 nected by cross bars, but on approaching the 

 upper part or roof of the ray they cross in all 

 directions and form an irregular network, the 

 intervals of which are occupied by softer inte- 

 gument. The ribs and bars are made up of 

 small pieces joined by plane but oblique sur- 

 faces, a mode of construction calculated to 

 admit of their being lengthened and shortened 

 upon one another, and thus to allow of the ca- 

 vity they surround being dilated and contracted. 



Fig. 9. 



Portion of a ray of Asterias rubens viewed laterally. 



A broad calcareous disk is situated on the 

 upper surface of the body, in the angle be- 

 tween two of the rays, (Jigs. 12 and 16, z,) which 

 is connected internally with a singular organ 

 named by Tiedemann the sand canal, to be 

 afterwards described. The calcareous pieces 

 are of a homogeneous structure, without cells 

 or fibres ; they consist, according to Hatchett's 

 analysis, of carbonate of lime, with a smaller 

 proportion of phosphate of lime. 



The coriaceous membrane which connects 

 the pieces of the skeleton is made up of white 

 glistening fibres. It is contractile and irritable, 

 for it slowly shrinks on being scratched with 

 the point of a knife, or when it is cut through. 



The external membrane is much thinner and 

 softer than that just described ; in various parts 

 it is coloured, or in these parts there is a co- 

 loured layer underneath it. 



The appendages or processes on the surface 

 of the body are of three kinds. First, calcareous 

 spines ; these are found over the whole surface 

 except the grooves for the feet. They are at- 

 tached by a moveable joint at their base to the 

 calcareous pieces of the skin, and are invested 

 by the external soft membrane nearly as far as 

 their point. Those on the upper surface are 

 solitary, short, and for the most part club- 

 shaped, their broader summit being marked 

 with radiating points ; whence they were named 

 stelliform processes by Tiedemann. On each 

 side of the groove for the feet the spines are 

 thickly set (c, c, Jig. 7) ; these in Asterias 

 rubens form three rows, in the middle and 

 innermost of which they are placed three deep. 

 On this part of the surface they are also longer 

 and pointed. The spines are slowly moved at 

 the will of the animal. 



The appendages of the second kind are of a 



very singular nature ; they have the appearance 

 of pincers or crabs' claws in miniature (Jig. 298, 

 c, 6, />, p. 615, vol. i.) and were described 

 by Miiller as parasitical animals under the 

 name of Pedicellaria. Monro gave the name 

 of antennae to analogous organs which are 

 found on the sea-urchin. They probably do 

 not exist in all species, for Tiedemann makes 

 no mention of them in his description of A. 

 auranliaca. In A. rubens they cover the 

 surface generally, and form dense groups round 

 the spines. Each consists of a soft stem 

 bearing at its summit, or (when branched) at 

 the point of each branch, a sort of forceps of 

 calcareous matter not unlike a crab's claw, 

 except that the two blades are equal and similar. 

 When the point of a fine needle is introduced 

 between the blades, which are for the most 

 part open in a fresh and vigorous specimen, 

 they instantly close and grasp it with consi- 

 derable force. The particular use of these 

 prehensile organs is not apparent ; their stem, 

 it may be remarked, is quite impervious. 



The third sort of appendages consists of those 

 which are named the respiratory tubes; they 

 will be considered afterwards. 



The other genera of Asteroidea have also a 

 cutaneous skeleton presenting the same general 

 mode of construction as that of Asterias, but 

 with certain modifications of structure and still 

 greater differences of form in particular cases. 

 Of these we may here notice the crinoid echi- 

 nodermata and the genus comatula, as the 

 most interesting examples. The former ani- 

 mals, comprehended by most naturalists in the 

 genus Encrinus, are, with one exception fthe 

 Enc. caput medusa or Pentacrinite) found only 

 in a fossil state, and the remains of their ske- 

 letons constitute the fossils named encrinites, 

 trochites, entrochites, &c. An idea of their 

 structure may be obtained if we imagine an 

 asterias placed with its mouth upwards on a 

 columnar jointed stem, one end of which is 

 connected to the dorsal surface of the animal, 

 and the other most probably fixed at the bottom 

 of the sea. The rays or arms extending from 

 the circumference of the body are much 

 branched, and at last pinnated ; other jointed 

 processes, named auxiliary arms, surround the 

 stem in whorls placed at short intervals. The 

 column is perforated in its centre with a narrow 

 canal, down which a prolongation of the sto- 

 mach extends, and lateral canals proceed from 

 the central one through the verticillate auxiliary 

 arms. The Comatula has rays spreading from 

 the circumference of the body, branched and 

 pinnated like those of the pentacrinite. It is 

 not fixed on a column, but the dorsal surface 

 of the body is elevated in the middle, and 

 bears a number of smaller rays or arms, and 

 this dorsal eminence with its rays has been 

 sometimes compared to a rudiment of the 

 column of the pentacrinite with its auxiliary 

 arms. Besides the mouth there is an anal 

 opening on the ventral surface, situated on an 

 eminence near the margin.* 



b. In the sea-urchin the calcareous matter is 

 disposed in polygonal plates, which, being 



* Meckel, Vergl. Anat. ii. p. 31. 



