398 



COMPARATIVE ANATOMY 



CHAP. 



The pedicellariiB of the Echinoidea are always stalked, and three-, less frequently 

 two-, or four-, bladed. Two principal forms may be distinguished : seizing pedi- 



cellarise (Fig. 345 A and C) and glandular pedi- 

 cellarise (Fig. 345 B and Fig. 346). 



a. The seizing pedioellarise. — The form of the 

 blades varies greatly in details. They are some- 

 times long and slender (p. tridactylce, tetradactyla;), 

 sometimes spoon-shaped and toothed (p. ophioce- 

 phalfe, seu buccales, seu triphyllae), or in other 

 cases broadened out like leaves (p. trifoliataj). 

 Each blade is always supported by a skeletal 

 piece, which determines its general shape and the 

 special form of its teeth, hooks, etc. The stalk 

 also is always supported by an axial calcareous 

 rod, which sometimes penetrates the whole of its 

 basal half (p. tridactylfe), sometimes only reaches 

 a short way into the base of the stalk. 



The tridaotyle pedicellarise of Centroste- 

 phanus longispinus (Fig. 345 C) will serve to 

 illustrate the structure of the seizing pedicellarice. 

 The three slender blades are connected at their 

 bases, and on the sides turned to the axis of the 

 whole forceps, by three transverse adductor 

 muscles, each of which is attached ou the inner 

 (axial) sides of two neighbouring blades. The 

 three muscles together form a triangle. These 

 adductor muscles are counteracted by opening 

 muscles, which run down on the outer sides of the 

 bases of the blades longitudinally. A nerve enters 

 each blade, running towards its tip, and innervat- 

 ing the musculature and epithelial sensory cells. 

 The inner surface of each blade is ciliated. Within 

 the stalk, the supporting calcareous rod reaches 

 only half way up, ending in a knob. The con- 

 tinuation of the calcareous rod is formed by an 

 elastic pillar, which consists of modified connective 

 tissue, and is enveloped in a sheath of longitudinal 

 muscle fibres. This arrangement makes it possible 

 for the distal portion of the stalk with the head 

 to bend in various directions, and even to bend 

 right back upon the basal portion. When the 

 muscles which bring about such movement are 

 relaxed, the distal part resumes the upright 

 position by means of the elastic pillar it 

 contains. 



The adductor muscles of these pedicellarise 

 consist of transversely striated muscle fibres ; 

 consequently these tridactyle pedicellarise are very active seizing organs. 



b. The glandular pedicellarise have been carefully investigated, up to the present 

 time, only in a small number of Echinoids {Sphmrechinus granulans, Ecliinus aciitns, 

 E. melo, DorocidaTis papillata, Strongylocentrotns Hvidus, Echinocardium flavasccus), 

 but it is probable that in time they will be found more widely distributed. In 

 general structure they resemble the ordinary seizing pedicellariEe possessing three 



Fia. 340.— Organisation of a glan- 

 dular pedicellaria of Sphaerecliinus 

 granularis, section. 1, Distal tactile 

 prominence ; 2, aperture of the glaud 

 of the forceps ; 3, proximal tactile 

 prominence ; 4, adductor muscle ; 

 5, skeletal piece of the forceps ; 6, epi- 

 thelium of the forceps ; 7, cavity of 

 the gland of the forceps ; 8, epithelium 

 of tlie same ; 9, muscle layer of the 

 forceps gland ; 10 and 11, opening 

 muscles ; 12, nerve ; 13, calcareous rod 

 in the stalk ; 14, aperture of the stalk 

 gland (16) ; 15, epithelium of the gland. 

 (The distal tactile prominence here 

 represented is wanting in this species.) 



