320 CRUSTACEA. LEPTOSTRACA 



as is the case in the Ostracods and many Phyllopods. In 

 front of the carapace is a movable plate or rostrum (a). 

 There are twenty-one segments in the body — five in the 

 head, eight in the thorax (r — t), eight in the abdomen 

 (u — I), the last segment (or telson) carrying two pointed 

 processes — the caudal fork (I). There are nineteen pairs 

 of appendages, as in the Malacostraca : the head bears 

 two pairs of antennae (c, d), one pair of mandibles (n), two 

 of maxillae (q, o) ; on the thorax there are eight similar 

 pairs of limbs (f) which are leaf-like and resemble those 

 of Phyllopods ; the abdomen has six pairs of appendages, 

 the first four being large biramous swimming legs (g), the 

 last two small and uniramous (h, k). The two posterior 

 segments are without appendages. The eyes are com- 

 pound and stalked. The mandible bears a long, three- 

 jointed palp (e). The anus opens on the telson between 

 the two branches of the caudal fork. 



The Leptostraca agree with the Malacostraca in having 

 the abdomen and its appendages clearly marked off from 

 the thorax ; in the position of the genital apertures ; in 

 possessing eight segments in the thorax ; in having nine- 

 teen pairs of appendages ; and in the occurrence of a 

 masticatory stomach. They differ from the Malacostraca 

 in the bivalved carapace with an adductor muscle ; in the 

 possession of leaf-like thoracic legs, and of eight abdominal 

 segments with a caudal fork. From most of the Mala- 

 costraca they are further distinguished by the presence of 

 a movable rostrum, and by all the segments of the thorax 

 being free. The group of the Malacostraca to which the 

 Leptostraca seem to be most nearly allied is the Mysidae 

 — a family of the Schizopoda. 



In the characters of the carapace and of the thoracic 

 legs, and in the presence of a caudal fork ? the Leptostraca 



