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ARACHNIDA. 



cetaceans is nearly cylindrical in shape, and remarkable for a series of raised rings, 

 which surround it like the hoops upon a barrel. As the creature increases in age, 

 it also increases in length, and adds ring after ring, in proportion to the depth of its 

 imbedment in the skin. The Burrower-barnacle is found in great numbers, and actually 

 studs the whale's skin with its shells. Not only does the skin suffer from their 

 presence, but the blubber is also infested by them, as they often pass completely 

 through the skin, and sink deeply into the fatty tissues beneath. I have seen several 

 fine examples of these sunken cirrhipedes, and could not but admire the wonderful ad- 

 aptation of their structure to their mode of living. 



CLASS ARACHNIDA. 



ANOTHER class of animated beings now comes before us, which, under the general 

 term of Arachnida, comprises the Spiders, Scorpions, and Mites. 



These beings breathe atmospheric air, they have no antennas, and they 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 Arachnida were placed among the insects, but may readily be 

 distinguished by several peculiarities. In the first place, they have more than the normal 

 number of 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 ; the body, thorax, and abdomen are merged 

 into one uniform mass, without 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 ( /. e. the jointed antennae- 

 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 is 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 the accompanying illustrations, several examples of these palpi are given, belong- 

 ing to Spiders which will be described in the course of the following pages. The reader 

 will not fail to remark their exceeding diversity of form, and the readiness with which 

 they can be distinguished from each other. They are all much magnified, as the 

 largest specimen is hardly equal to the head of a minikin pin. Still, their structure is 

 not very difficult to be comprehended, and a moderately good magnifying glass will 

 mostly be sufficiently powerful to answer the purpose. The Spiders all breathe by means 

 of certain lung-like organs, called the pulmonary sacs, though some species are also 

 furnished with air-tubes. These sacs communicate with the external air by means of 

 small apertues called " stigmata," which are analogous to the spiracles of insects. 

 There are seldom more than two of these stigmata, and never more than four. 



