ARACHNIDA. 6^3 



the Goose Barnacle, but are sessile on the rocks. Spots over which the 

 tide runs only for a few hours are thickly studded with these Barna- 

 cles, and it is interesting to see how quickly they open their valves and 

 fling out their arms as soon as the water covers them at each return- 

 ing tide. When the sea withdraws they close their shells firmly, and 

 retain within their interior a sufficiency of water wherewith to carry 

 on the business of respiration until the next tide brings a fresh supply. 

 Total submersion seems to be hurtful to them. 



ARACHNIDA. 



Another class of animated beings now comes before us, which, under 

 the general terra of Arachnida, comprises the Spiders, Scorpions, and 

 Mites, 



These beings breathe atmospheric air, have no antennae, and have 

 four pairs of legs attached to the fore parts of the body. 



In some of the higher Arachnida there is a bold division into thorax 

 and abdomen, and the former portion of the body is clearly divided 

 into separate segments. By the earlier naturalists the Arachnidse 

 were placed among the insects, but may readily be distinguished by 

 several peculiarities. In the first place, they have more than six legs, 

 which alone would be sufficient to separate them from insects. They 

 have no separate head, the head and thorax being fused, as it were, 

 into one mass, called the cephalothorax. In many of the lower species 

 there is not even a division between the thorax and abdomen, and the 

 body, thorax, and abdomen are merged into one uniform mass, with- 

 out even a mark to show their several boundaries. They undergo no 

 metamorphosis like that of the insects, for, although the young Spiders 

 change their skins several times, there is no change of form. 



Beginning with the true Spiders, we find that their palpi — i. e., the 

 jointed antennse-like organs that project from the cephalothorax — are 

 more or less thread-like, and in the males are swollen at the extremity 

 into a remarkable structure, as indicative of the sex as the beard of 

 man, the curled tail-feathers of the drake, and the gorgeous train of 

 the peacock. In the different genera these palpi are differently formed, 

 and afford valuable indications for systematic zoologists. 



In these strange creatures the mandibles are furnished with a curved 

 claw perforated at the extremity, sometimes like the poison-fang of a 

 venomous snake, and used for a similar purpose. A gland furnishes a 

 secretion which is forced through these organs, and is injected into any 

 object that may be wounded by the sharp claw. The fluid which is 

 secreted for the service of the fangs is nearly colorless, and is found to 

 possess most of the properties that exist in the venom of the rattlesnake 

 or viper. 



