254 PHYLUM ECHINODERMATA. 



mode of termination of the stone-canal, as the following state- 

 ment shows. 



The stone -canal opens to the exterior by a single pore in Pelagothuria 

 and in species of the following genera of Elasipodidae, Scotoplanes, Kolga, 

 Parelpidia, Elpidia, Peniagone, Benthodytes ; it opens to the exterior by 

 more than one pore in the following genera of Elasipodidae, Benthodytes, 

 Psychropotes, Laetmogone, Iliodaemon ; it ends blindly in the body wall, 

 opening into the body -cavity by several pores close to its blind end in 

 species of the following genera of Elasipodidae Irpa, Elpidia, Oneirophanta, 

 Orphnurgus, Benthodytes, and in the molpadian genera Trochostoma 

 and Ankyroderma. In other Molpadiidae, in Synaptidae and Dendro- 

 chirotae it opens into the body-cavity as in the last named, but is without 

 the blind part and the connexion to the skin ; lastly in the Aspidochiro- 

 tae the numerous pores lead from the body-cavity into a sac with 

 which the stone -canal communicates by one or more openings. It is 

 possible that this last arrangement gives the key to the explanation of 

 these strange variations in the termination of the stone-canal. As has 

 been fully described the stone-canal in other Echinoderms does not open 

 directly to the exterior, but into a portion of the body -cavity, the axial 

 sinus, which opens to the exterior by the water-pore or pores (madreporite) 

 and is derived from the anterior body-cavity of the larva. In adult 

 Holothurians there is apparently no trace of axial sinus or other derivate of 

 the anterior body-cavity. But in the larvae, as Bury has shown, a repre- 

 sentative of this cavity which has the appearance of being merely a small 

 appendage of the stone-canal (p. 152, Fig. 108) is present. It is possible 

 that the sac, into which the stone-canal of the Aspidochirotae opens and 

 the small dilatation into which the body-cavity pores lead in some other 

 forms, is the representative of the anterior body-cavity, which in other 

 Holothurians is so much reduced that it is not even discernible as a 

 dilatation on the stone -canal in the adult. On this view the pores of the 

 so-called internal madreporite are secondary perforations in the septum 

 which separates the much-reduced anterior body-cavity from the general 

 body-cavity, the real water-pore being aborted ; whereas in Holothurians 

 with an external madreporite, the water -pore of the larva has persisted, 

 but the anterior body-cavity into which it opens has become indistinguish- 

 able from the stone-canal. 



The appendages of the radial canals consist of prolongations 

 into the tube-feet and tentacles, and of some prolongations which 

 ramify and end blindly in the body wall. The tentacular canals 

 arise from the radial canals soon after their origin from the 

 ring canal (Fig. 179). They are provided (except in Elasipodidae) 

 with ampullae which project into the body-cavity. In the 

 Synaptidae alone do they arise direct from the ring canal. The 

 tube-feet prolongations arise alternately on each side of the 

 radial canals. Ampullae are always present and either project 

 into the body-cavity or are embedded in the body- wall between 



