ANNULOSA: CRUSTACEA. 297 



water in habit, and often found in great numbers in pools and ditches in 

 Europe. The different species of Branchipus (figs. 146 and 147, D) have 

 the transparent body unprotected by any carapace, and are found in ponds 

 and swamps in various parts of the world. The various "Brine-shrimps" 

 (Artemia) are found inhabiting the brine-pans in salt-works, or occur in 

 salt-lakes in both hemispheres, being especially abundant in Great Salt 

 Lake in Utah. 



In Estheria (fig. 147, G) the body is protected by a bivalve, sub-ovate 

 carapace, which is extremely like the shell of a Bivalve Mollusc, not only 

 in shape and appearance, but also in having the valves joined at their beaks 

 dorsally, and marked with concentric lines of growth. The species live in 

 fresh or brackish water. In Nebalia (fig. 147, C), the only marine type of 

 the order, there is a bivalved carapace, which is furnished with a beak or 

 "rostrum," and the eyes are pedunculated. There are two pairs of an- 

 tennse, and eight pairs of leaf-like respiratory feet, followed by a series of 

 natatory feet. There is no metamorphosis. Nebalia has decided affinities 

 with the Stomapods, and perhaps is not properly referable to the Phyllopoda. 



ORDER III. TRILOBITA. This order is entirely extinct, none 

 of its members having survived the close of the Palaeozoic 

 period. The Trilobites are Crustaceans in which the body is 

 usually more or less distinctly trilobed ; there is a cephalic shield, 

 usually bearing a pair of sessile compound eyes ; the thoracic 

 somites are movable upon one another, and are very variable in 

 number ; the abdominal segments are coalescent, and form a 

 caudal shield; there is a well- developed iipper lip or " hypostome" 



As regards the general structure of the Trilobites, the body 

 was protected by a well-developed shell or " crust," which 

 covered the whole dorsal surface of the body, and which usu- 

 ally exhibits more or less markedly a division into three longi- 

 tudinal lobes (fig. 148), from which the name of the order is 

 derived. The crust is composed of a cephalic shield, generally 

 crescentic in shape, a variable number of free and movable 

 rings, constituting the thorax, and a caudal shield or " pygi- 

 dium," the rings of which are more or less completely anchy- 

 losed. On the under surface of the head-shield in front, there 

 is situated a forked or oval upper lip or " labrum," which re- 

 sembles in form the labrum of the Phyllopodous genus Apus. 

 Recent researches by Mr C. D. Walcott have also consider- 

 ably increased our knowledge of the condition of the under 

 surface of the body in the Trilobites. This observer, namely, 

 has shown that the visceral cavity of the Trilobites (fig. 149, b) 

 was bounded inferiorly by a thin membrane, which is attached 

 to the lower margin of the dorsal crust all round. This ventral 

 membrane was strengthened by calcified arches, which in turn 

 supported the appendages beneath. As to these latter our 

 knowledge is not yet complete, but we know that in some forms 

 there existed a row of articulated appendages on each side of 

 the middle line below. The thoracic appendages seem to have 



