138 m. straus-durckheim's 



only covered by the carapon ; they may then be considered as 

 becoming internal ; but whatever may be their situation, they 

 are constantly in dependence upon the feet. 



In the Pulmonary Arachnid a, the organs of respiration 

 become really internal, forming more or less numerous sacs 

 which do not ramify, placed in corresponding groups in the 

 lower part of the body, communicating with the air by one 

 stigma for each group. 



In the Trachean Arachnida, and the Chilognatha, these 

 sacs are prolonged into long branching vessels, disposed in 

 tufts around the stigmata without communicating one with 

 another. 



In Insects, and the Chilopoda, the trachean system has 

 reached its highest development, the tracheae arising from each 

 stigma all communicating with one another. 



The larvae of Ephemera, which live in water yet breathe 

 only air, have the stigmata furnished with long foliaceous 

 appendages, containing air, which absorbs the oxygen from the 

 water, and enables it thus to be conveyed throughout the system. 



The trachece of insects, Myriapoda and Arachnida, consist 

 of three tunics, of which the external is an extremely thin 

 colourless membrane, not fibrous in its texture. The second 

 is a thread of a stiff corneous texture, wound in a spiral. This 

 thread is commonly round, but sometimes flat ; when round, 

 its whorls are in general less regular than when flat, being 

 mostly separated by a void of double the width of the thread. 

 The same thread is continued throughout one branch ; and 

 when this sends off a lateral branch, the turns of the spire 

 simply separate to give room for its insertion. When the 

 trachece bifurcate, the original thread ceases, and each branch 

 has its peculiar one. 



The third tunic is a very thin, white, mucous membrane, a 

 mere prolongation of the integuments. 



[I shall not enter into any abstract of the Circulatory System; 

 the incorrectness of the view taken of it by M. Straus being 

 fully proved by the observations of Mr. Bowerbank; never- 

 theless, it is but justice to M. Straus to say, that his remarks 

 on this subject well merit attentive perusal ; and had I not felt 

 that I have already occupied too much space with a subject 

 that is not perhaps likely to be of general interest, I should 

 have given an abstract of this part as well as of the others.] 



