&84. 



80 Guide to the Invertehrata, 



Crustacea. In these the head and trunk (cephalothorax) is composed of thirteen 

 GALLERY segments ; the abdomen (or pleon) of six segments and a " telson" or 

 East Side tail-spine. The Malacostraca are divided into two orders — (1) the 

 Table- Edkiophthalma, in which the eyes are sessile or fixed on the 

 cases surface of the head, and the head and thorax are distinct; and 



(2) the PoDOPHTHALMA, in which the eyes are movable, being fixed 

 on a peduncle or stalk (hence called "stalk-eyed Crustacea"), and 

 the head and thorax are generally united to form a cephalothorax. 



(a) Edkiophthalma: 1. — Amphipoda. The animals of this 

 division are small in size ; the body is compressed laterally. They 

 are mostly found in the sea, or on its shores, where they abound. 

 The common " sand-hopper" is a good example. One form, named 

 Necrogammarus Salweyi, occurs in the Lower Ludlow, Shropshire ; 

 Gampsonyx fitnhriatu8 in the Coal-measures of Rhenish Prussia; 

 and Prosoponisous proUematicua in the Permian of Durham. Yarious 

 Tertiary Amphipods have been described. 



Fig. 139. Fig. 140. 



Falctga Carteri, H. Woodward. ^ga moxophthalma, Johnston. 



Grey Chalk: Dover. Recent: Moray Firth. 



2. — IsopoDA. The Isopods have seven pairs of walking - legs 

 on the seven thoracic segments, which never bear gills ; these 

 are always attached to the appendages of the pleon or abdomen. 

 Frearcturus gig as occurs in the Old E,ed (Devonian) of Hereford ; 

 Bostrichopus antiquus in the Devonian of I^assau. Tru'B Isopods are 

 known from the Jurassic rocks ( Cyclosphceroma trilohatum), North- 

 ampton, and from Solenhofen ; Archcsoniscus Brodiei, from the 

 Purbeck of Wilts and Dorset. A parasitic form {Bopyrus) occurs 

 fossil under the carapace of Palc&ocorystes, from the Greensand 



