2 



Mr. G. J. Romanes and Prof. J. C. Ewart. (liar. 24, 



municates by well-defined apertures with that portion of the body 

 cavity which Jies between the sinuses and the oesophagus, and which 

 is reached through the circular apertures between the sinuses of the 

 circular canal. Each, canal sinns has three other apertures in its 

 walls. It opens by a small round aperture into a radial canal, and 

 the two other apertures occur as minute slits, one at each side of the 

 orifice of the radial canal leading into the adjacent tentacle sinuses. 

 When the tentacle into which the sinus opens is protruded, there is no 

 constriction between the sinus and the tentacle ; but when the ten- 

 tacle is retracted, there is a well-marked constriction at the junction 

 of the sinus with the ten-tacle. The e version of the perisome and the 

 protrusion of the tentacles are effected chiefly by the shortening of the 

 polian vesicle and the constriction of the longitudinal muscular bands, 

 which run from the inner surface of the body wall between each two 

 adjacent tentacle-sinuses ; but the circular fibres of the body wall also 

 assist in the process by contracting immediately behind the group of 

 sinuses, so as to act on them by direct pressure, and also indirectly by 

 forcing the body fluid against them. 



The amount of the body cavity fluid is constantly changing. At 

 the entrance to the cloacal chamber there is a circular valve which is 

 constantly dilating and contracting, except when the aboral end of 

 the animal is forcibly retracted. When open, this valve allows water 

 to pass into the respiratory tree ; when it begins to retract, water 

 escapes from the cloaca. This alternate opening and closing takes 

 place with perfect rhythm, at a rate of about six revolutions per 

 minute. At the end of every seventh or eighth revolution a large 

 stream of clear water is ejected, which sometimes contains sand and 

 the remains of food particles. When the tentacles are being pro- 

 truded, more water is taken in at the cloaca than escapes ; on the 

 other hand, retraction of the tentacles is preceded by an escape of a 

 large stream of water. 



In Echinus, two tubes spring from the under surface of the madre- 

 poric plate. The one is dilated at its origin, so as to include the 

 greater portion of the plate, and ends in the so-called heart; the 

 other is small, deeply pigmented, and runs along a groove in the 

 heart to open into a circular canal at the base of the lantern. From 

 the under aspect of this circular canal the five radial ambulacral 

 vessels take their origin. Immediately within the oral margin of the 

 shell, and alternating with the inner row of pedicels, are the five 

 pairs of "tree-like organs." If a fine glass cannula be forced through 

 the membrane which extends from the apex of each tooth to the oral 

 margin of the interambulacral plates and sides of the alveoli, coloured 

 fluids may be injected into the space between the membrane and 

 the alveoli of the lantern; the fluid then slowly diffuses upwards 

 into the vesicles around the apices of the teeth. It reaches these 



