NO. 9 A NEW ORDER OF CRUSTACEA PENNAK AND ZINN 3 



thorax and abdomen. The first four segments following the maxilliped 

 segment, however, bear appendages which presumably are comparable 

 with the thoracic appendages of the Copepoda. There are no signs 

 or rudiments of a pair of appendages which would indicate the fifth 

 thoracic segment. The maxilliped segment is so distinctive and unique, 

 and is so sharply set off from the adjacent regions of the body in 

 the Mystacocarida, that it may be conveniently termed a postcephalo- 

 some. The body of the Mystacocarida therefore may be divided into 

 a head, a single-segmented postcephalosome, a 4-segmented thorax, 

 and a 6-segmented abdomen. 



Except for the first and second antennae, the appendages of the 

 Mystacocarida differ markedly from those of the Copepoda (see 

 pis. I and 2). The mandibles are biramous and elongate, the first 

 and second maxillae are uniramous and elongate, and the post- 

 cephalosome appendages (maxillipeds) are irregular, broad, and flat. 

 The large size and the dense median setation of the relatively un- 

 specialized and primitive^ mouthparts indicate that they function in 

 straining food particles out of the interstitial water of sandy beaches. 

 The smaller mouthparts of the Copepoda, on the other hand, are 

 usually composed of relatively few segments and are more or less 

 specialized for the manipulation of food; this is especially true of 

 harpacticoid copepods which inhabit sandy beaches. Not only are 

 the trophic rami of the Mystacocarida large and unspecialized, but 

 also the head is proportionately much longer than it is in the 

 Copepoda. All of the thoracic appendages show a reduction and 

 simplicity found only in the fifth legs of a few copepods. Each leg 

 is a simple, unsegmented lamella which is probably useless in swim- 

 ming or crawling. 



Between the bases of the mouthparts of the Mystacocarida there 

 is an enormous, movable, broad, flat labrum. The labrum of the 

 Copepoda is an insignificant structure. 



On the posterior part of the cephalic shield, on the maxilliped 

 segment, on each thoracic segment, and on each of the first five 

 abdominal segments, there is a pair of lateral, dorsoventral, shallow 

 troughs with heavily chitinized, sculptured edges. Structures of a 

 somewhat similar nature are known in only one species of Copepoda 

 (Zinn, 1942). No function can be ascribed to them. 



Finally, the Mystacocarida may be differentiated from the Copepoda 

 on the basis of the location of the genital pore ; in the former group 

 it is on the first thoracic segment, while in the latter it is on the last 

 thoracic segment. 



