Pedicellarice in the Echinidas. 103 



corresponding processes of the companion jaw-pieces. Tlie 

 stem is rigid throughout, and contains a calcareous shaft that 

 traverses the entire length, and to which the head of the pedi- 

 cellaria is directly attached. Near the middle portion of the 

 stem is the large, inflated, gibbous, sac-like swelling pre- 

 viously mentioned ; and, considering its very prominent and 

 conspicuous character, it seems surprising that its occurrence 

 in this common Mediterranean species should not have been 

 more generally observed. 



The Glandular Sacculi of the Stem. — The description of 

 the pedicellaria will commence most conveniently at this 

 part. The sacculi of the stem-dilatation are generally dis- 

 posed in an irregular subspiral manner round the shaft, the 

 apices of the three being never, so far as I am aware, all 

 equidistant from the extremity of the shaft. After removing 

 the pedicellaria from the test and rendering the tissues suffi- 

 ciently clear for the transmission of light, it will be apparent 

 that each of the sacculi contains an elongate ovoid or sub- 

 cordiform mass, the upper portion of which terminates at the 

 pore. By this method of examination no vessels of any kind 

 are seen to open into the internal masses, either from above or 

 below, and they have the appearance of simply lying upon 

 the central rod and being held in position by attachment 

 around the foramen (PI. XII. fig. 4). 



On dissecting away a portion of the investing membrane 

 the mass can be readily displaced from within the sacculus 

 with a fine needle-point ; and it then has the appearance of a 

 white, spongy, reticulated substance with a denser central 

 portion within and a number of moderately large, elongated 

 pink cells distributed over its surface, and through the substance 

 itself, external to the central mass (fig. 3). In order to 

 arrive at the ultimate structure of this body recourse must be 

 had to other methods of investigation. I have found that 

 sections are unquestionably the best means of attaining this 

 end, although it is a very difficult matter to cut them of the 

 requisite thinness, in consequence of the hard and gritty cha- 

 racter imparted to the inner mucous mass by the chemical 

 action of the various preservative and staining reagents which 

 it is necessary to employ. On account of the nature of the 

 mucus, some fluids are altogether inapplicable ; picro-sul- 

 phuric acid or fresh water may be cited as examples ; and, 

 indeed, so sensitive does the mucus appear to be, that even 

 when specimens that have been preserved in alcohol for more 

 than a year are placed in water, the sacculi swell to many 

 times their normal size and burst in a few moments. 



I have found it to be the most satisfactory method of pre- 



8* 



