724 



SCIENTIFIC RECREATIONS. 



and house spiders. The sea spiders, though classed with the arachnida, are 

 sometimes placed amongst the Crustacea. We have the " tick " and the 

 cheese-mite and the scorpions, all of which belong to the spider family. 

 But the true spiders are known by the joining of the two upper segments, 

 the thorax and head being united (cephalo-thorax). The pretty and mar- 

 vellous webs are spun from abdominal glands through small apertures. 



The fluid hardens in its passage 

 sufficiently to be woven into threads 

 to resist the struggles of the captured 

 prey. The forms of these webs vary, 

 but some spiders do not catch their 

 victims in the net; they pounce upon 

 them cat fashion. The large house 

 spider is well known to all. The 

 garden spider is seen in the illustra- 

 tion (fig. 855) with the scorpion. 

 The habits of spiders will be found 

 a very interesting study, and many 



"S^^ft* ~ x volumes have been devoted to them. 



The water spider is a frequent 



Fig. 855. Arachnida. i. Spider (Epeira diademci) ; 2. Scorpion 

 (Scorpio.) 



inmate of an aquarium, and- the 

 bubble of air he takes down with 

 him to breathe serves as a means of 



living while he is seeking his aquatic prey. 



We will close our rapid survey of the invertebrate animals with a glance 



at the MOLLUSCA, which are divided into six classes (see page 703). The 



first is the Tunicata, which have no shell or hard covering, and come under 



the denomination of molluscoids, and belong to a lower order. The true 



mollusca include the Brachiopoda, which have a pair of shells. They are 



called " arm-footed " because a long 



cord or tendon passes through one 



of the shells, and fixes the mollusc 



to the rock. The Lingula of this 



class have been discovered in very 



old formations such as the Devonian 



period, and indeed appear to have 



been amongst the first created 



animals. 



The Lamellibranchiata include 



the oyster, cockle, mussel, etc. 



They are well known, and scarcely 



need description. The Pteropoda 



have no shell. The Gasteropoda are very numerous, and periwinkles, whelks, 



snails, etc., belong to this class. They progress by a muscular " mantle," 



which is extended and contracted. The " horns " have eyes at the 



Fig . 85 6.-Moiiusca. i. Nautilus (A >?*/) ; 2 . 



