STUDIES ON THK ARACHNID ENTOSTKKNITE. 251 



The opinion put t'orwiird by Lankester in 1881 (5) that the 

 entosternite was foi'uied by the enlargement and interlacing 

 of muscular tendons was modified in 1885 (7) by the further 

 suggestion that the prosomatic and smaller posterior (mesoso- 

 matic) entosternites are merely the original subepidermic 

 connective tissue of the sternal surface of the segments in 

 which they occur, which has become thickened and carti- 

 laginoid, and, in the case of the prosoma, has been at the 

 same time floated off, as it were, from the sternal surface, 

 taking up a position deeper, that is to say nearer the 

 axis of the animal than that which it oi'iginally occupied. 

 " And, again, in both Limulus and Scorpio the proso- 

 matic entosternite or plastron represents the mid-sternal 

 area of several segments fnsed, probably in both cases of 

 all the prosomatic somites, though possibly in Scorpio the 

 first segment is not included." The position of the ento- 

 sternite above the nerve-cord is explained on the hypothesis 

 that the detachment of the mass of connective tissue from 

 the sternal surface occurred at a period when the nerve cords 

 were still quite lateral in position, their union taking place 

 after the flotation. 



This hypothesis assigns an immense antiquity to the 

 entosternite, an antiquity dating back probably to the 

 Trilobitic stage of Arachnid phylogeny, possibly earlier still. 

 But supposing that the entosternite owes its origin to the 

 detachment of subneural fibrous thickenings of connective 

 tissue, a later phylogenetic stage can be ascribed to it by 

 assuming its derivation from paired thickenings which floated 

 off on each side of the united nerve-cords, and subsequently 

 fused with one another both transversely and longitudinally 

 to form a gate-like framework beneath the digestive tract. 

 May be the fenestration of the entosternite of Thelyphonus 

 is a survival of this early stage. But whether the entoster- 

 nite had its origin in subneural thickenings of this nature, 

 and, if so, the manner and purpose of their assumption of 

 their present position, or whether it was derived from the 

 fibrous solidification of muscular and connective tissue in the 



