V. Echinoidea. 23 



enlarges upwards and finally passes into a narrow ureter. This unites with the 

 stone-canal to form a common vesicle, which communicates by a narrow canal 

 with the collecting tubules of the madreporite. Lateral extensions of the central 

 cavity pass into the connective-tissue stroma around it, and are lined by vesicular 

 cells which are usually in several layers and frequently have parietal nuclei, as 

 in the renal cells of Helicidae. In the peripheral parts of the lobules the cells 

 lose their vesicular character and become filled with a finely granular mass con- 

 taining refractile particles. Fine canals pass outwards from the lobules which are 

 lined by a regular epithelium and open into the coelom on the ciliated free sur- 

 face of the kidney, 2 or more sometimes having a common opening or nephro- 

 stome. Groups of the refractile wander-cells are abundant in these tubes and 

 also pigment-granules. The connective-tissue stroma contains blood-spaces which 

 are in connection with those of the oral blood-vascular ring. The water- 

 vascular ring, which has no Polian vesicles attached, is not a simple canal, 

 but consists, like the lower end of the stone-canal, of many ^intercommunicating 

 diverticula lined by a low epithelium and surrounded by a connective-tissue 

 stroma containing blood-spaces. This latter constitutes the oral blood -vascu- 

 lar ring, and like that of Dorocidaris seems to correspond to the peripharyngeal 

 plexus of the Holothurians. Prouho's processus glaudulaire is an upward 

 extension of the kidney beyond the origin of the ureter. It consists of numerous 

 convoluted tubes which open on its surface into a large space surrounding it, but 

 not communicating with the exterior ; and towards the madreporite this space is 

 itself enclosed by a spongy tissue. The ciliated funnels in the coelom of Synapta 

 which are described by Leydig as in communication with the water-vessels are 

 perhaps isolated nephrostomes, and the terminal apertures of the lungs in other 

 Holothurians may be of the same character. In any case, however, the kidney 

 of the Urchins must be regarded as a part of the water-vascular system, and is 

 probably developed out of a more primitive excretory organ, like that of the im- 

 segmented worms. The interambulacra bear double lines of spines, tipped with 

 iridescent poison-capsules. Each spine is pierced by rows of pores and its 

 sharp tip is enclosed in the poison-sac. This contains a clear fluid with suspended 

 particles derived from its internal epithelium ; and it is enclosed in a capsule of 

 connective-tissue containing pigment-cells and muscular fibres, the latter being 

 attached above to the poison-sac and below to the shaft of the spine. When 

 they contract, the sac is withdrawn from the tip of the spine and the poisonous 

 secretion is discharged through the pores of the latter. Round the ambitus of the 

 test are some non-iridescent poison-capsules in which the musculature is but 

 slightly developed. At certain points on the stems of the poison-capsules the 

 epidermic cells are in relation with contractile connective-tissue elements, and 

 slow pulsations of the epidermis are visible. These structures are of the same 

 nature as the ampullae described by Prouho at the bases of the secondary spines 

 in Dorocidaris [see Bericht for 1887 Ech. p 15] and probably serve as lymphatic 

 hearts. In Aspidosoma as in Diadema there are spaces between the epidermic cells 

 of the perisome which open externally by fine canaliculi terminating in the cuti- 

 cular pores ; and the coelomic fluid is thus placed in osmotic communication with 

 the exterior. The Echinothuridae are related both to the Diadematidae and to 

 the Cidaridae, but are probably more embryonic and phylogenetically older 

 than either group. They are imbricated in the same manner as the Palaeechini- 

 dae, which is also the case in young Cidaridae. The isolation of the primary 

 ambulacral plates is an embryonic character, which is shared with the Cidaridae. 

 The apical system with the genital and ocular plates in one ring is like that 

 of the Palaeechinidae and Diadematidae, while in the Cidaridae there are 2 



