loo PHYSIOLOGY OF THE INVERTEBRATA. 



view: ( i) it is a means by which the spider obtains a livelihood ; 

 (2) it is subservient to propagation of the species * — the silk 

 being used as a cocoon for the reception of Q^'g-^^ a nest for 

 the young, as well as forming aeronautic gossamer lines for 

 the dispersion of the young brood on the approach of 

 maturity ; (3) in the genns Hydrachna (belonging to the 

 Acarina) it serves to attach the moulting individual to an 

 aquatic plant by the anterior part of the body, when it 

 struggles to withdraw itself from its exuvium ; (4) it forms a 

 home for the spider. 



The secretion of the salivary glands of Trijcuoria i/orncstiac 

 (the common house-spider) contains a diastatic ferment and 

 sulphocyanates. These were proved by the tests previously 

 given. 



The so-called "liver ducts" of Trrjcnuria domcstica have 

 been investigated by Mr. A. Johnstone, F.G.S.,t and the 

 author,]: with the following results : — 



When examined microscopically these ducts are seen to 

 consist of cellular tissue ; and the secretion is poured into the 

 intestine. The secretion obtained from a large number of 

 animals, as well as an extract made of the intestines of a very 

 large number of spiders, gave the following reactions : — 



(1) The secretion and extract form emulsions with neutral 

 oils, yielding subsequently fatty acids and glycerol. 



(2) The secretion and extract decompose stearin with the 

 formation of stearic acid and glycerol : — 



C..H.,„0, + 3H,0 = SC^AP, + (:,H,(),. 



(3) The secretion and extract act upon starch paste with 

 the formation of dextrose. The presence of dextrose was 

 proved by the formation of red cuprous oxide with Fehling's 

 solution. 



(4) The secretion and extract dissolve coagulated albumin 

 with the formation of peptones, which are readily recognised 



* See Dr. H. C. McCook's American Spiders and their tSpinniiu/ ]VorIc. 

 t Demonstrator in Geology in the University of Edinburgh. 

 t Proceedinf/s of Boy(d Society of Edinburgh, vol. 15, Ji. 113. 



