898 



COMPARATIVE ANATOMY 



The pedicellariiC of the Echiuoidea are always stalked, and three-, less t'rei[uentl}' 

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



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

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



ft. The seizing pedicellarise. — The form of the 

 Iilades varies greatly in details. They are some- 

 times long and slender (p. tridactyla", tetradactylse), 

 sometimes spoon-shaped and toothed (p. ojihioce- 

 plialte, sen buceales, sen tri^jhyllae), or in other 

 cases broadened out like leaves (p. trifoliatfe). 

 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 (]». tridactylre), sometimes only reaches 

 a short wa\' into the base of the stalk. 



The tridactyle pedicellarise of Centroste- 

 phanus longispinus (Fig. 345 C) will serve to 

 illustrate the structure of the seizing pedicellaria'. 

 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 

 nruscles, each of which is attached on 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 modiiied connective 

 tissue, and is enveloped in a sheath of longitudinal 

 muscle fibres. This arrangement makes it possible 

 for the distal i)ortion of the stalk with the head 

 to bend in various directions, and even to bend 

 right back upon the basal jiortion. When the 

 muscles which bring about such movement are 

 relaxed, the distal part resumes the upright 

 jjosition by means of the elastic pillar it 

 contains. 



The adductor muscles of these pedicellarise 

 consist of transversely striated muscle fibres ; 

 consequently these tridactyle pedicellaria 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 {Sijlmrcchhius gramdaris, Echiims acutus, 

 E. melo, Dorocidaris papillata, Strong ijlocentrutiis lividus, EcMnocardium fiavesccns), 

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

 general structure they resemble the oidinary seizing pedicellariiE ])ossessing three 



Fio. 340.— Organisation of a glan- 

 dular pedicellaria of Sphserechinus 

 granularis, .sectiun. l, Distal tactile 

 proinineuce ; 2, aperture of the gland 

 of the forceps ; 3, proximal tactile 

 prominence ; 4, adductor muscle ; 

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

 thelium of the forceps ; V, cavity of 

 the gland of the forceps ; 8, epithelium 

 of the same ; 9, muscle layer of the 

 forceps gland ; 10 and 11, opening 

 muscles ; 12, nerve ; 13, calcareous rod 

 in the stalk ; 14, ajjerture of the stalk 

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

 (The distal tactile prominence here 

 represented is wanting in tliis species.) 



