Introdtiction to A nimal Morphology. 1 4 1 



or one of the ambulacral areas (s. internoe) ; or ma}- surround 

 the bod}' equatorially (s. marginales) ; or may divide the 

 sides uniting with the marginal semitse (s. laterales) ; or may 

 lie in front of, or around, the anus (s. sub-anales). Around 

 the mouth is a buccal membrane bearing pedicellarise (modi- 

 fied spines, Troschel), usually with three valves, and having a 

 calcareous, netted axis in a soft, mucous tissue, and with 

 simple muscle bundles uniting the branches. These may 

 be: — I. P. globiferoe — with a solid stiff axis and a globular 

 head like a closed three-leafed bud. 2. P. stereophyllse — 

 with a hollow axis and a round, calcareous head. 3. P. tri- 

 dentes — shorter, with a hollow axis, and serrated arms with 

 incur\'ed points. 4. P. ophiocephalse — with a longer pillar, 

 and leaf-like limbs. 



The ambulacral system consists of a circum-oral 

 ring, five pedunculated Polian vesicles, cavernous in 

 structure, with netted, unstriped fibres in the wall on 

 the membrane over the dental apparatus, and five 

 ambulacral canals, one between each row of holes, 

 while proximal processes pass from the ambulacral 

 canals to the buccal tentacles [Perrier). The tube- 

 feet maybe alike (ambulacra homoeopoda), or dissimilar 

 (a. heteropoda), and each has at its free end a radially 

 plaited sucking membrane, under which is a cal- 

 careous, netted rosette, and sometimes a still deeper, 

 calcareous ring. Arcuated and branched spicules 

 often abound in the wall of the pedicelli. Heteropodal 

 forms like Spatangus may have (a) simple feet, with 

 no rosette; (/3) sucking spiculigerous feet, with an 

 entire or digitate margin ; (y) sensitive feet with broad 

 pencil-like ends and flattened knobs, supported on 

 fine, rod-like styles ; (S) gill-feet with pinnated margin ; 

 (c) buccal pedicelli with no ring, and a rudimental 

 rosette. The stone-canal from the madreporiform 

 plate has its walls stiff with calcareous spicules, but 



