8 TEXT-BOOK OF ENTOMOLOGY 



origin from Limulus-like or eurypteroicl forms ; their coxal glands 

 are retained from their eurypteroid ancestors. The Arachnida prob- 

 ably descended from marine merostomes, and not from an indepen- 

 dent annelid ancestry, hence we have represented them in the diagram 

 on p. 3 as branching off from the merostomatous phylum, rather 

 than from an independent one. 



The characters in which arachnids approach insects, such as 

 tracheae and malpighian tubes (none occur, as a rule, in marine or 

 branchiate arthropods), may be comparatively recent structures 

 acquired during a change from a marine to a terrestrial life, and 

 not primitive heirlooms. 



Arachnida also show their later origin than merostomes by the 

 fact that their sexual glands are in most cases single, and though 

 with rare exceptions the ducts are paired, these finally unite and 

 open externally by a common single genital aperture in the median 

 line of the body, at the base of the abdomen (Fig. 3, C, D). In this 

 respect Limulus, with its pair of genital male or female openings, 

 situated each at the end of a papilla, placed widely apart at the base 

 of the first abdominal limbs, is decidedly more archaic. Unlike 

 Crustacea and insects, Arachnida do not, except in the mites 

 (Acarina), which is a very much modified group, undergo a meta- 

 morphosis. 



We see, then, that the insects, with the Myriopoda, are somewhat 

 isolated from the other Arthropoda. The Myriopoda have a single 

 pair of antennae, and as they have other characters in common with 

 insects, Lang has united the two groups in a single class Antennata; 

 but, as we shall see, this seems somewhat premature and unnecessary. 

 Yet the two groups have perhaps had a common parentage, and may 

 prove to belong to a distinct, common phylum. 



Not only by their structure and embryology, as well as their 

 metamorphosis, do the myriopods and insects stand apart from the 

 Arachnida and other arthropods, but it seems probable that they 

 have had a different ancestry, the arthropods being apparently 

 polyphyletic. 



There are two animals which appear to connect the insects with 

 the worms, and which indicate a separate line of descent from the 

 worms independent of that of the other classes. These are the 

 singular Peripatus, which serves as a connecting link between 

 arthropods and worms, and Scolopendrella (Symphyla). These 

 two animals are guide-posts, pointing out, though vaguely to be 

 sure, the way probably trod by the forms, now extinct, which led 

 up to the insects. 



