26o PHYSIOLOGY OF THE INVERTEBRATA. 



The Peototracheata. 



This order is represented by the genus Pcri;patus^ which 

 contains several species. These animals have the power of 

 "throwing out a web of viscid filaments when handled or 

 otherwise irritated." This viscid matter is secreted by two 

 large ramified tubular glands situated on the sides of the 

 digestive tube, and open externally by the perforations of the 

 oral papilte. Feripatns breathes by means of tracheee, hence 

 the reason that Prof. Huxley has referred the order to which 

 PiviiMtus belongs to the Arthrojwda. 



From these remarks it will be observed that respiration in 

 Pcripatus is on the insect-type — i.e., by means of tracheal 

 tubes ; but the other excretory organs differ from those of the 

 Inseda. In the Insccta the renal organs are the Malpighian 

 tubules, but no such appendages to the alimentary canal are 

 present in Feripatus. 



The kidneys are segmental organs or nephridia, like those 

 of the worms, but of a more highly complex type. There is 

 a pair of these organs in each segment. They open in- 

 ternally into the body cavity, and externally at the base of 

 the limbs. 



The Myriapoda. 



The intestines of the animals belonging to this class are 

 provided with Malpighian tubules which perform an ex- 

 cretory function ; in other words, they are physiologically the 

 kidneys. 



The Insecta. 



Before describing the excretory organs, it is perhaps 

 desirable that we mention certain secretions, and the organs 

 (as far as possible) which give rise to them. 



(a) The poison which certain insects secrete is a fluid 

 strongly impregnated with formic acid. In many cases this 

 fluid is secreted by a special gland, and poured into a re- 

 ceptacle connected with the sting {c.f/., in Ajns and Vcnpa). 



