208 



ARACHNIDA. 



suits of the labours of this accomplished na- 

 turalist. 



He finds that each of these simple eyes is 

 composed, 1st, of a cornea; 2dly, of a crys- 

 talline lens; 3dly, of a vitreous body; 4thly, 

 of a kind of chamber; 5thly, of a choroid; 

 6thly, of a retina. 



The cornea, as is shewn in jig. 94, which 



represents a vertical section of the eye, is 



smooth and con- 



Fig. 94. vex externally (,) 



its superficies pre- 

 a senting none of 

 those divisions 

 which characte- 

 rize the cornea of 

 the compound 



eyes of insects. 



The internal sur- 

 face is deeply 

 concave, and in 

 the hollow he- 

 misphere thus 



formed is lodged the anterior part of the crys- 

 talline lens. This body (6) is of a spherical 

 figure, of a hard and transparent texture, resem- 

 bling in these respects the crystalline lens of 

 Fishes. Posteriorly it rests upon but does not 

 penetrate the vitreous humour. The vitreous 

 humour (c) is composed of a granular, soft 

 material, is larger than the chrystalline, plano- 

 convex anteriorly, wholly convex behind. As 

 the crystalline lens rests upon without sinking 

 into the vitreous humour, there remains a cir- 

 cular channel or space filled with an aqueous 

 humour, to which the term chamber may be 

 appropriately given, and which may be com- 

 pared to the posterior chamber of the eye of 

 some of the vertebrata. 



The retina (e} is applied to the back part of 

 the vitreous humour, and is in some degree an 

 expansion of the optic nerve (g). It is lined by 

 a choroid, or membrane saturated with a co- 

 loured matter, or kind of pigmentum (/), which 

 is afterwards reflected over the anterior margin 

 of the plano-convex surface of the vitreous 

 humour so as to form there a sort of pupil, the 

 aperture of which exceeds the diameter of the 

 crystalline, but is less than that of the vitreous 

 humour. Such is the somewhat complicated 

 structure of one of the large eyes of a scorpion, 

 by the knowledge of which physiologists are 

 now enabled better to understand the mode in 

 which vision is effected in the arachnidans. 



Organs of secretion. We designate thus the 

 organs that emit outwardly a matter which is 

 sometimes liquid, and sometimes becomes con- 

 crete by contact with the atmosphere. The 

 position of these organs varies; in one case 

 they occupy the anterior part of the body, in 

 another they are observed at the opposite extre- 

 mity. The nature and properties of the matter 

 secreted is not less variable ; in some instances 

 it is an irritating or poisonous liquid which the 

 animal introduces by means of a more or less 

 sharp pointed hook into the interior of the body 

 to which it may be applied ; in other instances, 

 again, it is a substance which is at first in 

 a liquid state, but soon becomes solid in its 



Fig 95. 



passage through a sort of sieve, or, if I may be 

 permitted the comparison, a cullender pierced 

 with excessively minute holes. We shall treat 

 separately of these two kinds of apparatus. 



Of' the apparatus for secreting the irritating 

 or poisonous liquid. Every one knows how 

 quickly a fly that has been bitten by a spider 

 expires : the effect is instantaneous. It is by 

 means of the mandibulae or forciples that the 

 spider has inflicted the wound. These mandi- 

 bulae are each armed with a moveable and ex- 

 tremely sharp claw, fjig. 95, a,) near to the 

 point of which is a minute orifice (6), from 

 which there escapes a drop 

 of poisonous liquid, which 

 spreads itself over the whole 

 wound the instant that it is 

 inflicted. This orifice, which 

 from its minuteness is very 

 difficult to be perceived even 

 with a high magnifying pow- 

 er, communicates with a fine 

 or narrow excretory canal (c ), 

 situated in the interior of the 

 -d mandible and given oft" from 

 the true secreting organ. This 

 gland is lodged in the inter- 

 space of the muscles of the 

 thorax; it is in the form of 

 an elongated and slightly 

 curved vesicle, the parietes 

 of which have a singular 

 structure. Treviranus describes it as consisting 

 of filaments adhering together and united by a 

 membrane so as to resemble a spirally disposed 

 band. This structure presents, he thinks, some 

 analogy to that of the trachea of insects. Ly- 

 onnet, in his posthumous work, has described 

 this part somewhat differently : he considers 

 each little band as being composed of two sub- 

 stances, one fleshy, which contracts upon drying, 

 the other squamous, which is disposed like a 

 watch-spring, or rather like Archimedes' screw, 

 and which always remains in the same state. 

 He supposes that these fibres, upon contracting, 

 force the poisonous liquid into the excretory 

 canal. Such a construction is not, however, 

 necessary, since it may be readily conceived 

 that that vesicle, being placed in the midst of 

 very powerful muscles, it is sufficient that they 

 contract in order to its compression and the 

 consequent propulsion of the fluid contained 

 in its interior, which probably the parietes have 

 secreted. 



This apparatus appears to us to correspond, 

 by its position, to that which is termed, in in- 

 sects, the salivary apparatus, and in silk-worms 

 the silk-glands : it is even possible that the 

 poisonous fluid itself, mingling with the ani- 

 mal juices which the spider introduces by 

 suction into its stomach, serves to facilitate 

 digestion. 



Spiders are not the only animals of their 

 class that are provided with this kind of organs. 

 Scorpions have also a poison-apparatus, but in 

 a very different position. It is not placed in 

 the mandibles, but at the posterior part of the 

 body, in the last segment of the tail-like abdo- 

 men. Every one is familiar with that pyrifonn 



