a 



PHYLUM ECHINODERMATA 



419 



A 



the oral Bur&oe of each arm to its extremity. Near to, but not 



quite in the middle point of the aboral surface is the anal 

 aperture, absent is a few instances ; and on the same surface, 

 nearer the margin, between the two rays of the bivium in the 

 five-rayed Starfishes, is the madrepori$e;a finely grooved calcareous 

 plate perforated by a number of minute apertures. In some fossil 

 Starfishes it is situated on the oral surface. Sometimes instead of 

 one madreporite there are several. 



The wall of the body in the Starfishes contains a number 

 of calcareous ossicles, movably articulated together and connected 

 by bands of muscle, so that, though the body is firm, and in the 

 dried condition often quite 

 rigid, the arms are capable 

 during life of slow move- 

 ments of flexion and exten- 

 sion, enabling the animal 

 to creep through compar- 

 atively small fissures and 

 crannies. A special system 

 of ossicles — the ambulacra!, 

 ossicles — are arranged in a 

 double row along each am- 

 bulacral groove, the ossicles 

 of the two rows articulating 

 movably with one another 

 at the apex of the groove. 

 At the end of the arm the 

 two rows of ambulacral 

 ossicles end in a terminal 

 ossicle which supports the 

 unpaired tentacle. Spines 

 are invariably present, but 

 are sometimes confined to 

 the margins of the ambula- 

 cral grooves, in which position they are movably articulated with 

 the underlying ossicles. Tubercles take the place of spines over 

 most of the surface in many forms. In Astropecten the ossicles 

 of the aboral surface take the special form to which the term 

 paxillm is applied. Each paxilla is a plate which is produced into 

 a short rod, divided at its extremity into a number of radiating 

 processes. 



The tube-feet are arranged in a double row along each of the 

 ambulacral grooves, each connected through an aperture between 

 the ambulacral ossicles with an ampulla, or, exceptionally, with two 

 ampullae, situated in the ccelome. Each double row of tube-feet 

 terminates at the extremity of the arm in an unpaired appendage, 

 the tentacle, which is tactile and olfactory, and not locomotive in 



E E 2 



Fio. 33 '. Anthenea. View of or.il surface. 

 (After Sladen.) 



