CLA8S I ECHINOIDEA 269 



acral areas from I to V, Koman, and the interambulacra from 1 to 5, Arabic. 

 The enumeration passes from left to rightj revolving like the hands of a 

 watch, the specimen beirig viewed from below and the odd anterior ambulacrum 

 being III (Fig. 370). When viewed from above, the order of enumeration is 

 necessarily reversed (Fig. 434). Lov6n showed that the size and character of 

 the primordial ambulacral plates give data by which a sea-urchin can be 

 oriented in young regulär Echini, and usually in adult Exocycloida. He 

 showed that of these teil plates, the la, IIa, Jllb, lYa, Yb are larger ; on the 

 contrary the Ib, Hb, lila, lYb, Ya are smaller (Figs. 370; 377, A). 



The mouth opens into an Oesophagus which conducts into a capacious 

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

 around 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 ör sieve-like 

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

 restricted to genital 2, madreporic pores as a Variation may extend to additional 

 genitals or to ocular plates. 



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 con- 

 nected 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) which are extended through the 

 pores of the ambulacral plates. Dilation 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 pver .a pair of ambulacral pores. The tube-feet serve usually 

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

 but in many forms, especially those having petaloid ambulacra, they are 

 modified so as to be partly brauch ial in function. Sometimes the tentacles of 

 the same ambulacrum differ in shape, structure and function, as in Arbacia. 



Eespiration is apparently effected by Stewart's organs in certain Echini. 

 These organs are internal, five in number, and situated radially, they are 

 given off from the periphery of the lantern membrane and beneath the com- 

 passes. In the Cidaroida, Stewart's organs are frondescent; in the Echino- 

 thuriidae, vermiform or sausage-shaped. External branchiae or gills exist in 

 the Centrechinoida as outward extensions of the oral integument. They exist 

 as ten small or larger branched fleshy organs interradially situated. Their 

 presence is marked by indenting cuts in the basicoronal plates so that their 

 presence is recognizable in fossils where they exist (Centrechinoida). In 

 Clypeastroids and Spatangoids, as well as partially in some of the Centre- 

 chinoida {Arbacia), the function of respiration is maintained by modified 

 dorsal ambulacral tentacles which have lost their function as locomotive 

 organs. For distinction these are called ambulacral gills. 



The vascular System consists of a ring-like vascular plexus surrounding the 

 Oesophagus, and immediately underlying the circular ambulacral vessel. This 

 ring gives off five radial vessels, and also two others which send off branches 

 to the stomach and generative organs. The central nerve ring, with its five 

 principal nerves running down the rays, is external to the two other Systems. 



