276 CRUSTACEA. 



SECOND SUB-CLASS. MALACOSTRACA. 



These are higher Crustaceans in which the body consists 

 of three regions with a constant number of segments, five to 

 the head, eight to the thorax, and six to the abdomen 

 (except in forms like Nebalia, which have seven). The 

 terminal piece or telson of the abdomen is regarded by 

 some zoologists as a distinct segment. Apart from this 

 telson, and also the segment next to it in Nebalia, all the 

 segments bear paired appendages. More or less of the 

 thorax is fused to the head region, and the anterior thoracic 

 limbs are usually auxiliary to mastication. Two compound 

 lateral eyes and a gastric mill are always present. There is 

 an antennary excretory gland, probably comparable with 

 the Entomostracan maxillary gland. (The female genital 

 apertures are on the third last pair of thoracic legs, the male 

 apertures on the last pair. Very few are hatched in the 

 Nauplius stage, many, however, at the Zoaea level, while 

 others have no metamorphosis at all. 



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, and link Malacostraca to 

 Phyllopods. The most 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-like 

 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, Ceratiocaris, &c. 



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. The 

 heart is elongated. 



