248 ARTHROPODS. 



Legion i. Leptostraca. Nebalia. 



Legion 2. Arthrostraca, with three orders, Anisopoda, 



Isopoda, Amphipoda. 

 Legion 3. Thoracostraca, with four orders, Cumacea, 



Stomatopoda, Schizopoda, Decapoda. 



Legion i. Leptostraca. 



Marine Crustaceans of great systematic interest, because they retain 

 in many ways the simplicity of ancestral forms. The mosi important 

 genus is Nebalia. 



A bivalve shell covers the whole of the lank body, except the last 

 four abdominal segments ; the head is free from the thorax ; the eight 

 segments of the thorax are free from one another, and the plate-hke 

 appendages resemble those of Phyllopods ; the abdomen has seven 

 segments and a telson with two forks ; the elongated heart extends into the 

 abdomen and has seven pairs of lateral apertures or ostia. Nebalia and 

 its congeners are probably related to certain ancient fossil forms from 

 Palaeozoic strata — Hymenocaris, Ceiatiocaris, etc. 



Legion 2. Arthrostraca. (Edriophthalmata, sessile-eyed). 



There is no shell-fold or shield except in the order Anisopoda. The 

 first thoracic segment (rarely with the addition of the second) is fused to 

 the head, the corresponding appendages serve as maxillipedes, the other 

 thoracic segments (seven or six) are free. The eyes are sessile. 



Order i. Anisopoda. The fusion of the first two thoracic segments 

 to the head, the presence of a cephalothoracic shield, and other 

 divergent features distinguish Tanais, Apseudes, etc., from the 

 Isopoda. 



Order 2. Isopoda. The body is flattened from above downwards. 

 The first thoracic segment is fused to the head, while the other 

 seven are free, and there is no cephalothoracic shield. The 

 abdomen is usually short, and its appendages, usually overlapped 

 by the first pair, are plate-like and function as respiratory organs. 

 The "wood-lice [Oniscm, Pomllio) are familiar animals 

 which lurk in damp places under stones and bark, and 

 devour vegetable refuse. Some related forms (c.f., Arma- 

 dillo) which roll themselves up are called "pill-bugs." 

 In the terrestrial forms there is obviously a departure from 

 the ordinarily aquatic habit of Crustaceans, but we do not 

 know that there is any special development of respiratory 

 structures. 

 Asellus is a very common form, living in both freshand salt water. 

 Ji/otea is not uncommon among the shore rocks 

 The " giihhXe." (Limnorin lignorum) is a destructive marine 



Isopod which eats into wood. 

 Among the marine Cymothoidfe which are often parasitic on 

 fishes, some, e.g. , Cymotha, are remarkable in their sexual 

 condition, for they are hermaphrodites in which the male 



