ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 253 



Echinodermata. 



EcMnoid covered witli Compound Eyes.* — Drs. C. F. and P. B. 



Sarasin have found at Trincomalee an echinoid (probably Diadema 

 setosum) covered with rows of blue spots ; one of tliese when examined 

 by the Microscope shows on the surface a mosaic of irregular hexa- 

 hedra or (more rarely) pentahedra, which are so disposed as to call 

 to mind the eye of an arthropod ; each polyhedron corresponds to a 

 pyramid of very highly refractive substance, the blunt end of which 

 is invested in pigment ; the pyramid is about 1/8 mm . long, and at 

 its base is 1/20 mm. broad ; there may be one hundred or as many as 

 one or two thousand pyramids in one spot. Over all of them the 

 body-epithelium forms a thin ciliated layer, which may be regarded 

 as the cornea ; each pyramid consists of a nimiber of vesicular cells 

 with quite hyaline contents, and in many of them there is a distinct 

 nucleus ; this region may be regarded as that of the lens and crys- 

 talline body. The distal surface and the distal parts of the lateral 

 surfaces of the pyramids are invested by a low epithelium, which 

 may be regarded as the matrix of the vesicular cells ; similar epi- 

 thelial cells are to be found at the proximal end, but the latter are 

 only found in the best developed eyes or those wliich lie nearest the 

 apical pole. All the pyramids have their lower half covered by a 

 layer of pigment, which is composed of pigmented connective-tissue- 

 cells ; the youngest eyes have, however, no pigment ; they are all set 

 on the nervous plexus of the skin, where there is a uni- or multi- 

 laminate ganglionic investment ; the nervous band is broken through 

 at various points. 



It is clear that the authors have discovered an optic organ com- 

 posed of a number of separate eyes, without indeed an optic nerve, 

 but directly placed on a ganglionic plexus ; many hundreds of these 

 eyes are present. If a hand is directed towards a point where 

 these eyes are developed the smToundiag spines are seen to turn 

 towards the spot ; even if the creature is only able to perceive li»ht 

 and shade, it does it so well as to have a very considerable organ of 

 defence in these eyes. 



The authors have been able to detect flask-shaped gland-cells in 

 the integument, and give a short account also of a system of vessels 

 which they have been able to make out. 



Wandering-cells of Echinoderms.t — Herr E. Metschnikoff, in 

 the fifth of his studies on comparative embryology, deals with the 

 wandering-cells of Echinids and Asterids ; they appear to be cells 

 which break off from the endoderm or the portion of the blastoderm 

 which forms that layer, and to pass into the cleavacre-cavitv where 

 they fulfil various functions. This mode of development agrees with 

 what is seen in adult sponges, and also with the mode of mesoderm- 

 formation seen in Acalephae and in Iihoj)aIonema ; it is a comparatively 

 low process, which attains a higher gi-ade in Ctenophora (see p. 256). 



* Zool. Anzeig., viii. (ISSo) pp. 715-20. 



t Zeitscbr. f. Wiss. Zool., slii. (1SS5) pp. 656-71 (1| pi.). 



