350 



PHYLUM ARTHROPODA 



female the ovaries may become quite degenerate. Forms allied to 

 Sacculina are grouped together as Rhizocephala. One of them — • 

 Sesarmaxenos — occurs on a fresh - water crab, Sesarma, in the 

 Andamans ; all the rest are marine. 



Second Sub-Class. Malacostraca 



Series I. Leptostraca. Division Phyllocarida. 



Marine Crustaceans of great systematic interest, retaining in many 

 ways the simplicity of ancestral forms, and linking Malacostraca and 

 Entomostraca. The most important genus is Nebalia (Fig. 193). 



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 Branchiopods ; 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. There 

 are both antennary and maxillary excretory organs. Nebalia and its 

 congeners are probably related to certain ancient fossil forms from 

 Palaeozoic strata, e.g. Hymenocaris from the Cambrian. 



Series II. Eumalacostraca. 



Division i. Syncarida, the order Ana- 

 spidacea, primitive fresh-water forms, 

 without a carapace ; with the eight 

 thoracic segments all distinct (Anaspides), 

 or with the first one fixed to the head 

 [Koonunga) ; with stalked eyes in Ana- 

 spides, sessile eyes in Koonunga ; with 

 lamellar branchiae on the thoracic legs, 

 whose slender exopodites are also res- 

 piratory (Fig. 194). 



Division 2. Peracarida, with a carapace 

 that leaves at least four of the thoracic 

 segments free, with the first thoracic seg- 

 ment always fused to the head, with 

 usually sessile eyes, with a brood-pouch 

 on the thoracic appendages of the female, 

 with an elongated heart, with direct de- 

 velopment. Numerous orders including : 

 — the pelagic Mysidacea (formerly united 

 with Euphausiacea as Schizopods), e.g. 

 Mysis ; the pelagic and deep-water 

 Cumacea, e.g. Cuma and Diastylis ; the 

 Isopods, with dorso-ventral flattening of 

 the body, a posterior heart, and respira- 

 tory organs on the abdominal limbs, e.g. 

 the terrestrial wood-lice {Porcellio, Onis- 

 cus, etc.), which show minute trachea-like 



Fig. 195. — An Amphipod 

 (Caprella linearis). 



The two anterior thoracic seg- 

 ments are fused to the head ; 

 the abdomen is greatly re- 

 duced and without append- 

 ages ; the fourth and fifth 

 thoracic segments bear only 

 respiratory plates. 



