SKELETO-TEOPHIC TISSUES AND COXAL GLANDS. 133 



lateral processes is identical ; only in Mygale we find a fuller 

 development, a repetition of well- marked processes correspond- 

 ing to successive segments of the body, which is not so well- 

 marked in Limulus. In Mygale, as in Limulus, there is no 

 subneural arch, nor is the gut nor the aorta involved in any 

 dorsal excrescence of the tendinous mass. 



An entosternite like that of Mygale exists in the common 

 British Epeira diadema. 



The tissue of which this " floating " skeletal plate is formed 

 has a very closely similar structure in all three genera. It is 

 represented in Pi. VII, where figs. 1 and 2 from Scorpio, fig. 3 

 from Limulus, and fig. 4 from Mygale are all drawn to the 

 same scale. 



Histology of the Entosternite of Limulus. — The 

 large size of the entosternite of the King Crab renders 

 an examination of the character of its tissue more satis- 

 factory than that of other Arachnida. When cut through 

 with a knife, the substance has much the appearance of Verte- 

 brate hyaline cartilage ; it is of the same consistence and 

 translucence, and is, in fact, superficially so much like Verte- 

 brate cartilage that one is led to expect the presence of the 

 same chemical bodies in its composition. This expectation is, 

 however, fallacious. My colleague, Prof. Schafer, F.R.S., has 

 been kind enough to make a chemical examination of this 

 tissue, and he reports that it does not yield chondrin nor gela- 

 tin, but nearly equal quantities of chitin and of mucin, with a 

 very small quantity of albuminate.^ 



' Professor Schiifer has furnished me with the following additional note : 

 — "The reasons I have for thinking that the substance you placed in my 

 hands may probably contain * chitin ' are chiefly negative. After removal of 

 the mucin the residue is insoluble in almost everything I have tried except 

 concentrated sulphuric acid, and iu this it does not dissolve at all readily 

 without the aid of heat. It swells in alkalies, apparently without becoming 

 dissolved, even on heating. The solution in concentrated sulphuric acid, dropped 

 into boiling water and kept boiling a short time, yields a substance iu solution 

 which dissolves cupric hydrate in presence of caustic potash, but on boiling 

 the mixture I was not able to obtain much evidence of the formation of 

 cuprous oxide, although I once or twice have thought 1 could detect traces. 



