Crustacea — Ostracoda. 



71 



segments referred to the head and thorax (cephalothorax), whilst GALLEEY 

 the remaining six belong to the hinder part of the body called j.^g^ g^'^^ 

 the abdomen (or pleon). Owing to the hard external covering -^all-cases 

 possessed by nearly all the class, and to the fact that they mostly 13 & 14, 

 lead an aquatic existence, they are very abundantly represented cases 

 in a fossil state in many geological deposits. 80-85. 



(I) Entomostraca. — In the older formations in which Crustacea 

 are met with, they all belong to the section Entomostbaca, in 

 which the body-segments are usually indefinite in number, and the 

 limbs are very varied in character, the most important being attached 

 to the head, the bases of the limbs serving also as mouth-organs. 



1. — OsTEAcoDA. In the Ostracoda the entire body is enclosed in 

 a shelly carapace, composed of two valves united along the back 



Fig. 120. 



-Small bivalved forms of Crustacea : Cladocera, Ostracoda, 

 and Copepoda. 



1. Daphnia pulex, Linn, 2. Candcna hispida, Baird. 



3. Cyclops quadricornis, Linn, a, an adult female ; b, c, d, larval stages. 



Figs. 1, 2, and 3 are all fresh-water forms. 



U- Cetochilus sepfentrionalis, Goodsir. Firtli of Forth. 



5. Sapphirina ovatolanceolata, Dana. Atlantic. 



6. Nicothoe Astaci, Aud. and Edw., parasitic, found on gills of the lobster. 



7. Nauplius stage of a Copepod. 



by a membrane ; the valves can be closed or opened at will, and 

 the appendages, with which the animal can either creep or swim, 

 are protruded from the lower border. These small crustaceans 

 first appear in the Lower Cambrian ; they are met with subsequently 

 in rocks of almost all ages, and are equally abundant living to-day 

 both in seas and lakes. They must be looked upon as one of those 



