TYPE EGHINODERMA. 577 



hsemal canals, as the radial schizocoelic sinuses have been 

 termed in this group, as in the Ophiurids. These canals con- 

 sist of five tubes lying between the radial bydrocoel-canals 

 and the radial nerve-cords, and terminating blindly at their 

 oral extremities by coming into contact with the peripharyn- 

 geal partition ; they are not continued within the partition and 

 there is no circumoral sinus. The lacunar system consists of 

 five radial lacunae, lying in the perihsemal canals, penetrating 

 into the peripharyngeal space, where they unite into a circular 

 circumoral lacuna (?r), from which branches pass to the walls 

 of the digestive tract and which is in connection with the 

 lacunae of the ovoid gland. This structure, as stated, lies in 

 the wall of the axial sinus, and, as in other forms, stands in 

 close relationship to the reproductive organs, its lacunae being 

 continued into the walls of the genital cords. 



The hydrocoel has the usual arrangement of a pericBso- 

 phageal circular canal (Jir) from which five radial canals pass 

 off {rJi), each terminating in a tentacle {tt) perforating an 

 ocular plate. From the perioesophageal ring the stone-canal 

 {se) passes aborally to open into the axial sinus close to the 

 madreporiform tubercle, and in addition in the radial Echi- 

 noids the ring has attached to it in each interradius a spongy 

 structure which is usually termed a Polian vesicle {pv), 

 though these structures in other groups are saclike. The 

 tube-feet (tf) which perforate the ambulacral plates are in the 

 majority of forms, and especially in the radial ones, very 

 extensible and provided at the tip with a sucking-disk, and so 

 assist the spines in locomotion. Two pores as a rule exist 

 for each foot ; through one of these the branch issuing from 

 the radial canal passes, and through the other a branch passes 

 back from the foot into the interior of the body to terminate 

 in a saclike ampulla. The feet, however, near the aboral 

 surface are frequently branched and lack a sucker, serving a 

 respiratory function rather than a locomotor, and in the 

 bilateral Echinoids, in which frequently the tube-feet occur 

 only on the aboral surface of the test, nearly all the feet may 

 assume a tentaclelike or pinnate form and become respiratory. 



The digestive tract in all those forms in which the mouth 

 occupies the centre of the oral surface is provided with a 



