ARTHROPODS OF DOUBTFUL POSITION. 81 



The little that is known about the embryology of the group does not throw any 

 light on their systematic position, but the facts that are known of their anatomy would 

 seem to indicate that the position of these forms may be near the lower mites 

 (Acarina). The eggs are very large, and when they are laid the parent casts its 

 integument, which covers them in a manner somewhat like that of the ephippia of the 

 Entomostraca described on a preceding page. The egg undergoes total segmentation, 

 and the young hatch with four, or occasionally three, pairs of appendages. 



The water-bears are not very common, but should be looked for in Avater squeezed 

 from the damp moss of swamps. Like the rotifers which occur with them they are 

 said to withstand desiccation, the addition of water to their dried bodies reviving 

 them. There are several genera, Mdcrobiotus^ Milnesium, and Echiniscus being the 

 most prominent. No fossil forms are known. 



SUB-CLASS. GIGANTOSTRACA. 



Only one living genus remains to illustrate this group which once formed a very 

 prominent feature in the fauna of the world. Naturalists are now pretty well agreed in 

 the union of the trilobites, horseshoe crabs, etc., in a group to which Professors Haeckel 

 and Dohrn have applied the name Gigantostraca, and Dr. Packard the name Paleocarida. 

 To the latter student belongs the credit of first clearly showing the close relationships 

 existing between the Trilobita and Merostomata. Of the extinct forms of course but 

 little can be known. Geologists can learn but little of the anatomy or embryology of 

 the fossil forms, and hence we must depend largely upon analogies drawn from the 

 study of Limulus for our knowledge of the trilobites. 



ORDER I. TRILOBITA. 



There are but few groups of fossil animals which possess more popular interest than 

 those peculiar forms known as trilobites. This arises not only from the beauty of their 

 form, but from the questions which have arisen in regard to the position which they 

 should occupy in the systems of classification. Linna3us described the few which were 

 known to him as EntomolitJms paradoxus, thus clearly indicating his belief , that they 

 were fossil articulates. Latreille, struck by their superficial resemblance to the chitons, 

 thought to range them near those forms among the Mollusca. Usually, however, they 

 have been regarded as Crustacea, and have been assigned varying positions within that 

 group, some considering them as allied to the Phyllopoda, while others placed them 

 near the isopodan genus, Serolis. More recently they have been considered as near 

 relatives of the horseshoe-crab, and together with that problematical form they have 

 been shifted about, at one time being regarded as Crustacea, at another as Arachnida, 

 or as occupying a place between these two groups. 



This uncertainty has arisen partly from the fact that we know almost nothing of 

 the structure of the trilobites, and until recently but little of the anatomy of Limulus, 

 and less of its development. 



The name Trilobite, which means three-lobed, is applied to certain articulated 

 animals, in which the body is usually divided into three well-marked regions : first, a 

 head, rounded in front and usually bearing large compound eyes ; second, a thorax made 



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