262 



ABTHROPODA 



[CH. 



1. Coxa. 



"maxilla." 3. Trochanter. 4. Femur. 

 5. Patella. 6. Tibia. 7. Tarsus. 

 8. Palpal organ. 



the limb-like form of the appendage and so produce a true 



gnathite. 



On the ventral surface 

 of the abdomen just behind 

 the waist is situated the 

 genital opening, protected by 

 a plate ; on each side of this 

 is the slit-like orifice of a 

 lung-book. The lung- books 

 are very remarkable struc- 

 tures. Each opens to the 

 exterior by a pore through 

 which the air enters, and 

 1 consists of a sac the cavity 



FIG. 115. Pedipalp of Tegenaria guyonii, O f which is largely occupied 

 the large house-spider. , , ,, . , , 



by a number of thin plates 

 2. Gnathobase, the so-called j n the substance of which 



the blood circulates and is 

 thus brought into close rela- 

 tionship with the air which 



passes in and out between the neighbouring plates ; the sac is 

 floored in by a special plate which is a modified appendage (Figs. 116 

 and 117). Such a breathing apparatus is peculiar to the Arachnida. 

 In some Spiders we find a second pair of lung-books placed behind 

 the others, and in other species this second pair is replaced by a 

 pair of tracheae recalling the respiratory mechanism of the Myria- 

 pods or the Insects (Fig. 116). They have however been indepen- 

 dently developed, and probably owe their origin to the sac of a 

 lung-book from which the lamellae have disappeared. Certain 

 branches of these tracheae situated near the middle line are be- 

 lieved to be modified tendons of the abdominal muscle, for in all 

 Arthropoda the tendons of the muscles are formed by hollow 

 involutions of the ectoderm lined by chitinous cuticle. 



Near the hinder end of the abdomen are four tubercles or 

 spinnerets, and if these be pushed aside, two more, 

 shorter in length, come into view. These are the 

 organs which form the web and they have been shown to be vestiges 

 of abdominal appendages. They are very mobile and are pierced 

 at their ends by hundreds of minute pores through which the 

 silk exudes as a fluid, hardening on exposure to the air (18, 

 Fig. 118). 



Spinnerets. 



