50 THE BRITISH NATURALIST. [March 



This tendency to coalesce, which is also shown in the anterior somites 

 of the Myriapods, is more pronounced in the three higher classes ; the 

 head of the fly, as well as the head of the spider which kills it, 

 undoubtedly represents a number of coalesced somites, the abdominal 

 and thoracic regions of most Arachnidans and insects present similar 

 features. The tendency to develop spinning glands, results of which are 

 evident in the beauty of the spider s web, in the cocoon of the silk-worm, 

 and in the less perfect cocoons of other insect larvae, is present in the 

 Myriapods, many of which spin webs. 



Again in this central type we find a foreshadowing of the poison 

 gland and fangs which in the spiders become such formidable weapons. 



Existing Myriapods show clearly how the mouth organs of insects and 

 Arachnidans have been developed from walking legs. Curious though it 

 may appear, it is nevertheless true, that the jaws of insects are modified 

 legs ; organs originally of locomotion, they have become modified for 

 biting or sucking. The early stages of this process are indicated in 

 Peripatus, whose second somite bears a pair of mandibles the structure of 

 which is simple, and places beyond doubt the fact that they are walking 

 legs modified for biting. The third somite also bears a pair of modified 

 legs usually called the maxillae. 



We can well understand how an animal, which had one or two pairs 

 of legs in the vicinity of the mouth, so modified as to enable it to get a. 

 greater quantity or variety of food, would possess a decided advantage in 

 the struggle for life, and transmitting this favourable variation to its 

 progeny, the process would tend to become accentuated, and would lead 

 to the development of such organs as the mandibles and maxillae of the 

 Myriapods, and ultimately to the development of the chelicerae and 

 pedipalpi of the Arachnidans on the one hand, and the mandibles and 

 maxillae of insects on the other. 



Existing Myriapods have the pair of legs on the fourth somite further 

 modified into a second pair of maxillae, which are homologous to the 

 labium of insects. The origin of this remarkable transformation of legs 

 to jaws may be easily understood by anyone who will watch a cockroach 

 feed. The action of the jaws of this insect may be described as follows : 

 The mandibles, the organs of the second somite, seize as well as bite the- 

 food, the maxillae, the organs of the third somite, not only bite but push 

 the food into the mouth, showing how these organs have retained their I 

 ancient function of helping to bring food to the mouth. It is worthy of 

 notice that young Myriapods grow by addition of segments posteriorly in 

 a manner very suggestive of many worms, and the reverse of that, process, 

 of coalescence which shows itself in adult Myriapods, and more strikingly: 

 in the higher classes. In some species of Myriapods also there is a tendency 

 for some of the tracheae to be aggregated and open on the dorsal surface,,) 

 in a manner suggestive of the pulmonary sacs of spiders and scorpions. 



