ECHINODERM AT A PEDICELLARIA 399 



seizing blades, which are opened and closed by special muscles, but the fibres of the 

 adductor muscles are not transversely striated. In the stalk, the axial calcareous rod 

 is continued as far as the three-bladed head, an arrangement which greatly decreases 

 the mobility of this kind of pedicellaria. 



The most distinctive characteristic of these pedicellarise, however, is the 

 presence of a large glandular sac in each blade. This glandular sac, which, as is 

 shown by recent discoveries, consists of two fused sacs, causes each blade to be pear- 

 shaped. It is covered by a thick glandular epithelium, and has a muscular wall of 

 its own, in which the (smooth) fibres run in circular layers. This muscular wall, no 

 doubt, serves for pressing out the slimy, and probably poisonous, secretion, through 

 the aperture which lies near the tip of the blade. This aperture appears in most 

 cases to lie 011 the outer side of the blade. 



At the base of each blade, on its inner side, the epithelium is thickened to form 

 a tactile prominence or cushion, which (besides cilia) carries immovable sensory 

 hairs. In Echinus acutus, besides the basal, or lower, tactile prominence, there is, 

 on each blade, a distal or upper prominence, which also lies on the inner side of the 

 blade (Fig. 346). 



Numerous nerves pass from the stalk of the pedicellaria into its head and its 

 blades, so as to innervate the muscular and the sensory cells. 



In a few Echinoids, glands also occur on the stalk of the pedicellaria ; such 

 glands are specially strongly developed in Sphcercchinus granularis. These 

 glands, three in number, encircle the stalk of the glandular pedicellaria} (p. gemmi- 

 formes), about half way up. Each gland is a large vesicle with an aperture, through 

 which, on stimulation, a slimy secretion is discharged. The wall of the vesicle con- 

 sists of glandular epithelium within a muscular layer. The three glands cause large 

 vesicular swellings on the stalk of the pedicellarue on which they occur ; they are 

 covered by undifierentiated outer body epithelium. 



If we imagine that, in such pedicellaria} provided with stalk glands, the distal 

 portion of the stalk above these glands degenerated, or was no more developed, we 

 should have the form of pedicellaria which is called p. globifer. Such globifers, 

 occasionally still provided with rudimentary seizing forceps, have been discovered in 

 Centrostephanus longispinus and Sptuvrechinus granularis, side by side with ordinary 

 pedicellarise. They are capable of pendulous movements. 



The function of the pedicellarise has not yet been satisfactorily decided. The 

 view that, in Echinoids, they play some part in locomotion, has recently been 

 decidedly opposed, and it has been maintained that Echinoids move exclusively by 

 means of their ambulacral feet and spines. It has further been asserted that the 

 pedicellaricE lay hold of foreign objects, algae, etc., and hold them fast on the upper 

 side of the body in order to hide it, but this view also has been opposed, the function 

 therein ascribed to pedicellaria} being claimed for the tube-feet. Such a function could, 

 in any case, only be accessory. Another view is that the pedicellariae serve for the 

 holding of prey, and for carrying it to the mouth. In the Asteroidea, however, 

 which take the food in large pieces (Fish, Crabs, Mussels, Snails, Echinoids, etc.), 

 they could not well play this part. 



The most probable view is that the pedicellarise are protective organs, and fulfil 

 the function of cleaning the spine-covered body. They clear away foreign bodies. 

 Small animals which come into contact with the body are seized, and enveloped in 

 the slimy secretion of the epithelium, or in the possibly poisonous secretion of the 

 specialised glands of the pedicellaria}, arid held until they are dead, and then 

 " thrown overboard." In this way Echinoids and Asteroids may protect themselves 

 from animal and vegetable growths, parasitic or otherwise. This would explain the 

 astonishing cleanness of most members of this group in spite of their spinous 

 covering. 



