CLASS I 



ECHINOIDEA 211 



, covered ////// *>/'/ .s7/-//<7///vx, <//></ i-nrrifing spines. Mouth on the under or 

 nrfiim! *////'</.-/ : "/'"> *ihnili-'l i-illu-r in fin- ilufun-n /////// system or somewhere in tin- 

 iiiisti'rinr iiifrniiiiliiihii-riiiii. l'lnl<* <>f tin 1 ///'- (/////////("/"/ ureas more or less ex- 

 ti'ntinlii i>< rjr<itnl fnr lite protrusion <//' fn In- feet. 



Tl,,- y;.s/. The test or covering of Echinoids is composed of numerous 

 ealeareon> plates, tinnly united by their edges, so as to form a more or less 

 n'ui'l ease or box, and disposed in eertain regions or systems. In a few 

 instances, however, the plates overlap one another in an imbricating manner, 

 so as to impart a certain decree of flexibility to the test. The main element 

 of the test i> ailed the corona, and the system of plates at the summit is 

 known as the /n/-.s-n.v////v// or ii/iirnl X//N/V//,. 



The test is pierced by two large openings, which are surrounded by 

 membranes studded with small calcareous particles. One of these is the 

 ni''iifh, which is always placed on the under or actinal surface of the body, 

 may be either central or eccentric in position, and is surrounded by a cori- 

 aceous peristomial membrane. The other aperture is the anus, surrounded by 

 a periproctal membrane. In some orders it is placed opposite the mouth in 

 the centre of the apical system, in which case the test is said to be regular or 

 endocydic. But in other orders it is situated beyond the apical system at a 

 variable distance in the median line of the posterior interambulacrum, either 

 dorsally or actinally ; the test is then said to be irregular or exocyclic. 



The mouth opens into an oesophagus, which conducts into a capacious 

 stomach, and thence into a convoluted intestine. The digestive tract winds 

 round the interior of the test, being attached to the inner surface of the 

 latter by muscles, and terminates in the anus. Surrounding the oesophagus 

 is a circular vessel filled with water, which is admitted by the so-called 

 stone canal, opening externally in a madreporite. This is a porous or sieve-like 

 structure, consisting of a variable number of canals, and though commonly 

 restricted to one of the plates of the apical system, it may extend backwards 

 and perf orate a large part of the system. 



The circular vessel gives off five branches, known as the radiating canals, 

 which pass along the ambulacral areas on the interior of the test, and connected 

 with it in the interambulacral areas are five distensible membraneous reservoirs, 

 termed the Polian vesicles. The radiating canals give off numerous lateral 

 branches or tube feet (tentacles, pedicels) which are capable of protrusion through 

 pores of the ambulacral plates. Emission is effected by means of secondary 

 vesicles or ampullae which by contraction force their contained fluid into the 

 tube feet and distend them. The ampullae, as a rule, communicate with the 

 tube feet by two canals perforating the plates separately, a single tentacle 

 being placed over a pair of ambulacral pores. The tube feet serve normally 

 as locomotive organs, when they are prehensile and end in a suctorial disk ; 

 but in many forms, especially thos'e having petaloid ambulacra, they are 



XIV.), 1882-84. Schliiter, C., Die regularen Echiniden der norddeutschen Kreide (Abhandl. zur 

 geolog. Special-Karte von Preussen, Bd. IV.), 1883. Idem, Neue Folge, Heft 5, 1892. de L<>ri<>h 

 P., Description des Echinides (Commission des travaux geol. du Portugal, vol. II.), 1887-88. 

 Lwtn, S., On Pourtalesia (Svensk. Vetensk. Akad. Haudl., Bd. XIX.), 1884. Pomel, N. A., Class- 

 ification methodique et genera des Echinides vivantes et fossiles, 1883. Duncan. P. M., Revision 

 of the Genera and Great Groups of the Echinoidea (Journ. Linn. Soc., vol. XXIII.), 1889. Ebert, 

 7*., Die Echiuiden des nord- uud mittel-deutschen Oligocans (Abhandl. zur geol. Spec. Karte von 

 I'reussfii, Bd. X.), 1889. Loven, S., Echinologica (Bihang till Svensk. Vetensk. Akad. Handl., 

 Bd. XVIII.), 1892. Clark, W. B., Mesozoic Eohinodermata of the United States (Bull.- U.S. Geol. 

 Survey, No. 97), 1893. 



