20 Echinoderma. 



basis of the test and spines and traverse them by diapedesis. The pores of the 

 madreporite in Dorocidaris all lead to a single internal opening at the apex 

 of the water-tube. This is lined by columnar epithelium, and lying alongside it 

 is another tube, the canal aquifere annexe, which is lined by a pavement 

 epithelium and opens below into the ovoid gland. It unites with the water-tube 

 not far below the madreporite. No currents could be detected passing through 

 the madreporite in either direction, and water seems to enter the ambulacral 

 system by simple diffusion only. The histological structure of the tentacles is 

 the same as in the Echinidae. In the connective-tissue layer are numerous curved 

 spicules with small projections which are more developed near the sucker. The 

 terminal spicules form a complete ring, but lower down they do not extend over 

 the line of the tentacular nerve, beneath which the thickness of the connective 

 tissue and elastic layers is much reduced. All the tentacles are relatively small, 

 and towards the apical pole their suckers decrease in size, finally disappearing 

 completely. The water-vessels contain the same amoeboid corpuscles as the 

 coelom. They enter each tentacle by the outermost of its two pores and are carried 

 along to its end by ciliary action, returning to the water-vessel by the inner pore. 

 The canals of the visceral lacunar system in D. as in E. are not true 

 vessels, but only interstitial lacunae hollowed out in connective tissue. But one 

 marginal lacuna, the external, can be definitely traced on the intestine. Attached 

 to its internal border is a wide meseuteric fold containing a large channel which 

 communicates with the external lacuna by numerous ramifications. The latter, 

 when full, is cylindrical, but has no lining epithelium, though otherwise well de- 

 fined. The internal lacuna is irregular and indistinct, but extends rather nearer 

 the rectum than the external one. The capillary network is well defined along 

 the first coil of gut, becoming less so on the second, until finally the canaliculi 

 are so small as to be no longer traceable. The inner lacuna joins the oral ring 

 which, in E. as well as in Z)., sends branches over the ovoid gland to the genital 

 pentagon, and before reaching it they are joined by a network from the external 

 lacuna. The glandular tubes described by Perrier on the mesentery of Echinus 

 sphaera between the madreporite and the stomach are parts of this lacunar 

 system. The oral ring sends 5 branches down the pharyngeal crests which soon 

 open into a wide system of lacunar spaces, and these again pass into the radial 

 lacunae which are situated in the connective tissue beneath the external faces of 

 the water- vessels and have hitherto escaped notice. Both in E. and in D. they 

 give off branches which accompany the tentacular water-vessels. The perineural 

 space or schizocoel between the nerve and the blood-lacuna is closed in both 

 genera, not communicating either with the coelom or with its fellows, nor forming 

 an oral ring. The cavities of the visceral lacunar system contain the usual amoe- 

 boid cells, and also a finely granular amorphous substance not found elsewhere. 

 The brown corpuscles are abundant, specially in the internal marginal vessel of 

 old individuals. - - The ovoid gland or axial organ contains a central cavity lined 

 by epithelium and closed below ; but its upper end is continued into the caual 

 aquifere annexe which joins the water-tube immediately beneath the madre- 

 porite. The periphery of the gland contains blood-lacunae, being continuous 

 with the tissue of the oral ring. Its surface is pierced by fine canals which pass 

 inwards from the exterior and are lined by a continuation of the external epithe- 

 lium. The more central portion also contains canals surrounded by alveoli of the 

 connective tissue which are filled with the colourless, refractile, and brown 

 amoeboid cells, but chiefly the former, and also brown pigment granules which 

 are more abundant with age. Towards the upper end of the gland a second ca- 

 vity gradually becomes developed with a sort of axial cord, the processus 



