and Ayiatomij of the Echinodermata. 377 



stone-canal takes its origin, ascends perpendicularlj upwa'rds, 

 and opens outwards through the pores of madreporic plates. 

 The latter possess no arrangement bj which they can be 

 closed ; they are rather always open for the entrance and 

 exit of the sea-water on the one hand, and of the fluid con- 

 tents of the water- vascular system on the other. 



The sanguiferous cavities consist of the following parts : — 

 In the first place the five longitudinal canals and the annular 

 space enveloping the nerve-ring. In the Echinida these 

 structures have nothing to do with the true blood-lacunse ; 

 the latter originate as ventral and dorsal intestinal lacunae 

 from the blood-lacunar ring which lies upon the surface of 

 the lantern. From the dorsal intestinal lacuna branches 

 ramify which run to the glandular organ (the so-called heart 

 of previous writers) and surround it. At its terminal portion 

 (it extends into the body-wall and, indeed, into the schizocoele- 

 sinus of the anal pole) lacunfe of the anal blood-lacunar ring 

 are in connexion with this organ. This lacunar ring runs in 

 an annular schizocoele-sinus surrounding the anus, partly 

 projecting into it^ partly in its wall ; from it blood-lacunae 

 are given ofi" to the sexual organs. 



Peculiar organs are the five vesiculiform lobate structures 

 situated upon the surface of the lantern, and previously 

 described as Polian vesicles. From the water-vascular ring a 

 canal leads into them, opening into their cavity, while blood 

 moves in the connective wall m lacunai which stand in direct 

 communication with the blood-lacunar ring. 



In the Spatangida there are present the five longitudinal 

 canals and the oesophageal sinus communicating with them. 

 The true blood-Jacunar ring has, however, disappeared with 

 the lantern, and both the dorsal and ventral intestinal lacunae 

 open into this oesophageal sinus, in which the nerve-ring is 

 situated, and which has been designated the blood-lacunar 

 ring. The dorsal lacuna, however, runs beside an intestinal 

 water-vessel, which latter originates from the annular canal, 

 which likewise concentrically surrounds the buccal aperture. 

 This water- vessel and the intestinal lacuna communicate with 

 each other in their further course, and run along the gland 

 until the true stone-canal, originating from the madreporic 

 plate, enters into the web of vessels produced by the amalga- 

 mation. 



In this way is produced a communication between the 

 water- vascular system and the blood-lacunar system, and thus 

 between cavities of entodermal and schizocoelar origin, such 

 as occurs in no other group ot Echinodermata. That this 



