MALACOSTBACA. 447 



Jatter pierce the body of the host, and carry nourishment to the 

 parasite. Mantle saccular, and without calcareous plates, with 

 narrow aperture which can be closed. Mouth and alimentary canal 

 absent. The testes are usually paired, lie between the ovaries, and 

 open into the brood-pouch. The EhizocepJiala live principally as 

 parasites on the abdomen of the Decapoda, and wind their root-like 

 filaments around the viscera of the latter. 



Fam. Peltogastridae. Peltog aster pay uri Eathke. Sacculina carcini Tliomps., 

 LerncBodiscus porcellana Fr. Mull., Brazil. 



II. MALACOSTRACA. 



The Malacostraca differ from the Entomostraca in possessing a 

 constant number of segments and paired appendages. The boundary 

 between the head and thorax cannot be absolutely fixed on account 

 of the varying number of anterior pairs of legs which are modified 

 to form jaws. These regions are composed of thirteen segments 

 altogether, and bear the same number of pairs of appendages, while 

 the abdomen, which is always distinct, includes six segments and the 

 same number of paired limbs and terminates with an anal plate 

 (telson) derived from the terminal portion of the body. 



Amongst the living Malacostraca there is, however, a single group 

 of forms (Nebalia) (fig. 355, a, b), which differ in having a larger 

 number of abdominal segments. They have, in addition to the six 

 abdominal segments with appendages, two segments without appen- 

 dages, and an elongated Phyllopod-like caudal fork. This remarkable 

 form was for a long time regarded as a Phyllopod, and in many of its 

 characters represents a connecting link between the Phyttopoda and 

 the Malacostraca. The structure and segmentation of the head and 

 thorax resembles that of the Malacostraca, but the terminal region of 

 the abdomen does not present the special form of a caudal plate or 

 telson. In Nebalia we probably have to do with an offshoot of the 

 Phyllopod-like ancestors of the Malacostraca, which has persisted to 

 the present time. 



The head includes in all cases, behind the mandibular segment 

 on which two paragnathi form a kind of underlip, the segments of 

 two pairs of maxillae. The latter preserve more or less the characters 

 of Phyllopod feet. The head, therefore, consists of five segments, each 

 with its pah* of appendages, viz., two pairs of antennae, one pair of 

 mandibles, and two pairs of maxillae. It is followed by the thorax, 



