650 AEACHNIDA. 



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 adaptation of their structure 

 to their mode of living. 



CLASS ARACHNIDA. 



ANOTHEK 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 antennae, 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 ; and the body, thorax, and abdomen are merged into one uniform 

 mass, without even a mark to show their several boundaries. They undergo no metamor- 

 phosis, 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 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, belonging 

 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 apertures called 

 " stigmata," which are analogous to the spiracles of insects. There are seldom more than 

 two of these stigmata, and never more than four. 



