NERVE-NET 



515 



cells which establish relay's with ganglion cells which have migrated inwards 

 from the surface layer and have formed an interlacing network ; through 

 these the underlying muscular layer is stimulated (Figs. 679 and 680). The 

 early histologists pictured such a net as a sjmcytium composed of anastomos- 

 ing fibres (Hadzi, 1909), but vital staining shows that the fibres run parallel 

 to and intertwine with each other without actual fusion (Bozler, 1927 ; 

 Woollard and Harpman, 1939) ; the junctions, however, are not polarized 

 as in true sjaiapses. Conduction, therefore, is free and equal in all directions 

 so that any stimulus is diffusely spread (Eimer, 1874 ; Romanes, 1876); 



Figs. 679 axd 680. — The Subepithelial Xerve-net of Ccelexterates. 



Fig. 679.— The nerve- 

 net of Hydra (after 

 Clause, Grobben and 

 Kiihn). 



Fig. 680. — Subepidermal nerve-net around the oral 

 disc of Hydra. 



E, epidermis ; G, ganglion cells in the nerve-net ; 

 S, epithelial sensory cells (after Hadzi). 



consequently if interdigitating incisions are made and the animal is cui/ 

 into zig-zag strips leaving only nervous comiections, impulses pass either 

 way and round corners so that an effective res2)onse is obtained. ^ 

 Indeed, if all the sense-organs but one are removed from a jellyfish, the 

 rhythmic impulses for swimming movements are started by the sole survivor 

 and proceed in all directions. Control is thus entirely peripheral and execu- 

 tion indiscriminate without evidence of central mtegration, and to any 

 stimulus the response is monotonously similar and universal. 



While perijjheral control by such a subepidermal nerve-net is the sole 

 mechanism available to Coelenterates, it persists in many animal groups — ■ 

 Echinoderms, worms (Fig. 681). Molluscs, and Balanoglos.sid.s — and finds 

 an analogy in the autonomous visceral plexuses of Vertebrates such as 

 the myenteric plexus ^\hich coordinates movements of the intestine. As 

 evolution proceeds, ho^\•ever, the nerve-net assumes a more and more sub- 

 sidiary role. The only area in turbellarian worms wherein the primitive 



' Jellyfish — Mayer (1908); sea-anemone — Parker (1917); colonial Coelenterates — Parker 

 (1920). 



