1881.] On the Locomotor System cf Echinodermata. 



3 



vesicles partly by passing directly upwards external to the alveoli, and 

 partly by passing into the cavities of the alveoli and ascending through 

 the circular sinus. 



In Spatangus the ambulacral circumoral canal has no polian vesicles 

 or sinuses developed in connexon with it. Some of the pedicels 

 have suckers, others are conical and devoid of them, while others 

 again are flattened at their tips, and sometimes split up into 

 segments. 



If one of the arms of Solaster papposa is divided transversely, and 

 a coloured fluid introduced into the open end of the radial canal, the 

 ampulla? and pedicels of the injected arm are at once distended. The 

 fluid next penetrates the circular canal, polian vesicles, ampullae, and 

 pedicels of the other arms ; but unless considerable pressure be kept 

 up for some time, none of the solution enters the madreporic canal. If, 

 however, the pressure is maintained for several hours with a column 

 of fluid 2 feet high, the fluid .ascends through the stone canal and 

 diffuses slowly through the madreporic plate. When a thin slice is 

 then shaved off the plate, the fluid is observed escaping from a 

 small circumscribed area situated between the centre and the margin 

 of the plate, and corresponding in size and position with the termi- 

 nation of the stone canal on the inner surface. The stone canal, 

 gradually increasing in diameter as it passes inwards from the 

 madreporic plate, runs obliquely over its accompanying sinus, till it 

 finally hooks round this sinus to open into the circular canal. 

 Springing from this canal and opposite to each inter-radial space 

 (with the exception of the space occupied by the stone canal) is a 

 polian vesicle. The size and form of these vesicles are largely de- 

 termined by the amount of fluid in the pedicels. In none of the in- 

 jected specimens was there any evidence of a communication between 

 the ambulacral vessels and the body cavity, or between the ambulacral 

 and the blood (neural) vessels. There was, however, abundant evi- 

 dence of communication between the latter and the exterior. When 

 a cannula was introduced into the outer end of the sinus, a coloured 

 solution could be easily forced through the sinus into the circular 

 blood-vessel, and from this into the radial blood-vessels. But when 

 the cannula was introduced into the proximal end of the sinus, the 

 solution rapidly rushed along the sinus and escaped through the 

 madreporic plate, proving that the blood-vessels of Solaster commu- 

 nicate far more freely with the exterior than do the water- vessels. 



The ambulacral system of the common star-fish only differs from 

 that of the sun- star in having no polian vesicles. Astrojoecten, on 

 the other hand, has polian vesicles : but in it the pedicels have de- 

 parted from the usual form in being short, conical, and unprovided 

 with terminal suckers. In Ophiura the pedicels are morphologically 

 similar to those of Astropecten, though shorter and more slender. 



b 2 



