V. Echinoidea. 13 



the blood lacuna to form a common vascular plexus with a fine epithelial lining, 

 which applies itself to the coelomic side of the axial organ or gland situated at 

 the end of the great diverticulum of the gut. This contains numerous canals which 

 are in free communication with those of the vascular plexus on its side ; all have 

 an epithelial lining and contain pigment both in irregular granules and in cells 

 which latter are also found in the stroma of the gland. The vascular plexus reaches 

 its greatest size about the middle of the gland, but becomes smaller again towards 

 the madreporite where the stone-canal opens into it. The pore-canals of the 

 madreporite lead into the stone-canal alone and into no other cavity, and on 

 its way down towards the vascular plexus it passes through a great pentagonal 

 schizocoel sinus which also encloses the upper end of the gland. The connective 

 tissue wall of this sinus is penetrated by blood-lacunae, which collectively represent 

 the anal lacunar ring of the regular Urchins. They are in connection with the 

 cavities of the axial organ and its vascular plexus and also send extensions over 

 the genital ducts. These ducts end in papillae which protrude externally through 

 the pores of the genital plates, and are lined by ciliated epithelium. 



The discovery by the cousins Sarasin ( J ) of compound eyes in Diadema seto- 

 sum was noticed in the Bericht for 1885 I p 191. The eyes appear in life as blue 

 specks of variable size, a large one being on each genital plate with smaller ones 

 on the periproctal plates. A line of large spots follows each iuterambulacrum and 

 forks with it, the spots becoming smaller towards the margin of the test and gra- 

 dually disappearing on the oral side. There is a median line of small spots in 

 each ambulacrum and a ring round the base of each interambulacral tubercle. In 

 an allied species the large interambulacral spots are fused into continuous lines. 

 Each spot consists of a variable number of pyramids of refractile substance with 

 their inner ends enclosed by cups of dark pigment. The blue colour results from 

 iridescence. The eye is covered by a sort of cornea composed of reduced epidermic 

 cells bearing a ciliated cuticle. The pyramids consist of a number of thick- walled 

 vacuolated cells the nuclei of which form a layer at the proximal and distal end 

 of each pyramid. Beneath the pyramids is a layer of nerve fibrils and cells, some 

 of the latter lying among the stellate cells of the pigment cups. The latter are 

 probably connected with connective-tissue fibres, bundles of which pass upwards 

 through the nervous layer, itself generally resting on a bed of black pigment. The 

 vacuolated cells of the pyramids are modified epidermic cells. - - In the ordinary 

 integument the ciliated cuticle is pierced by fine holes, leading into the duc- 

 tules of unicellular glands which occupy a subepithelial position together with 

 the sensory cells and pigment cells. Below these again lies the plexus of fine 

 nerve-fibrils, already described by Prouho [see Bericht for 1886 Ech. p 10]. 

 The apparent fibres which traverse this plexus vertically are not the lower ends 

 of supporting cells, but minute tubules which open externally through the cuticle 

 and originate in lacunae of the connective tissue beneath the nerve plexus. This 

 lacunar system contains blood corpuscles and probably constitutes the peripheral 

 termination of the blood- vascular system, which is thus brought into direct relation 

 with the external water as in Cyclas and many Worms. 



According to Prouho (') the first rudiments of the genital apparatus of Stron- 

 gylocentrotus appear in the mesenteric fold which surrounds the water-tube and the 

 ovoid gland. A ring is gradually developed beneath the madreporite and sends a 

 process into each interradius on which lateral outgrowths appear. No connection 

 is traceable, however, between this ring and the ovoid gland, analogous to that 

 described by Perrier in Antedon [see Bericht for 1886 Ech. p 5]. 



Prouho ( 2 ) finds that the so-called blood-vascular system of Urchins consists 

 mostly, if not entirely, of interstitial lacunae in the mesentery and its appendages. 



Zool. Jahresbericht. 1887. Echinoderma. e 



