QUEKETT'S LECTURES ON HISTOLOGY. 



83 



transparent flesh. The skeleton, as shown in fig. 140, is composed of 

 three calcareous jaws, having a sharp recin-ved tooth at the apex and an 

 internal serrated edge, while the tissue surrounding the jaws is strength- 

 ened by minute bicurvate spicula ; it is seated on a cylindrical stalk 

 placed in the centre of the fleshy stem. 



" All these parts, when highly magnified, present the characteristic 

 structure of the shell of the animal ; the soft tissue, on the contrary, is 

 transjmrent, contractile, and, like that of the cirrhi, is capable of con- 

 siderable elongation and flexion. While the Echini;s is living, the 

 Pedicellarias are always in active movement from side to side, the jaws 

 are continually opening and shutting, and if a small body be placed within 

 them, it is held with tolerable force. They are attached to the soft fleshy 

 covering of the shell by a dilated base, and are not confined to any par- 

 ticular part of the shell, but many may be seen on the thin membrane 

 closing the oral aperture. The part which I have called the stalk is 

 somewhat dilated at each extremity ; its structure resembles that of a 

 small spine, and it is stated by Sars that each stalk, like a spine, is 

 articulated to a minute tubercle ; but of the truth of this I have never yet 

 been able to satisfy myself, as in all cases after their removal the soft 

 stem has been found to completely invest the whole of the calcareous 

 matter. 



" If the Pedicellarije be removed from the Echinus, they will continue 

 in active movement for some time, and if one of them be touched with a 

 needle or pin, those in the neighbourhood will all bend towards the one 

 that has been irritated. In the Asteriada? the Pedicellarias are of a 

 diiferent form to those in the Echini — in the Asterias ruhens, for examjile, 

 in which they are very abundant, as shown at a a, in fig. 109, the 

 calcareous jaws are like the two valves of a 

 mussel, as represented at b, in fig. 140, two of 

 the edges being serrated, whilst the other two, 

 which arc not closely approximated, have a 

 semicircular notch, leaving an opening between 

 them when in apposition, and the stem is short 

 and flexible, but not provided with a calcareous 

 axis as in the Echinidaa. When magnified 

 130 diameters, as shown at c, the characteristic 

 reticulated structure is exhibited. Mounted 

 specimens, taken from the outer surface of the 

 shell of Echinus miliaris, as represented at a, 

 in fig. 140, show very distinctly the three jaw^s 

 and the axis or stalk, but being in a state of 

 contraction, the soft parts apjjear very short 

 and puckered up, so that a species of neck is 

 formed between the jaws and the axis ; this, 

 however, is not the case in living specimens. 

 All the parts composing the skeleton of the 

 Pedicellarias exhibit the characteristic reticu- 

 lated structure of the Echinodemiata. The 

 jaws are thin, flattened below, sharp above, 

 and bent nearly at right angles, so as to form a 

 tooth ; the axis is about 5th of an inch in 

 length and dilated at both extremities, and in 

 shape and structure is very like the spine of an Echinus. On either side 

 of the jaws may be seen a row of small bicurvate spicula, somewhat 

 resembling those in the disc of the cirrhi of the Echinida% but differing 

 from them, as represented at d, in Plate XIV., fig. 19, of the first volume 



G 2 



a, Pe licellana of Echinus 

 miliaris. h, skeleton of one of 

 the Pedicellaria'. of Asteria 

 rube7is. c, portion of the 

 skeleton of the same magnified 

 130 diameters. 



