2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. IO3 



tached to the head between the bases of the antennae and extends pos- 

 teriorly as far as the postcephalosome. The mandibles are biramous 

 and unmodified, while the first and second maxillae are uniramous and 

 elongate. The postcephalosome bears a pair of flat, broad, irregular 

 mouthparts (ordinarily corresponding to the maxillipeds of other 

 groups). Each thoracic segment has a pair of small, unsegmented 

 lamellar appendages. The terminal (anal) segment bears two large 

 caudal rami which have prominent setae. None of the appendages 

 are prehensile. The dense setation of the median margins of the 

 mouthparts indicates that the Mystacocarida probably feed by strain- 

 ing particles of food from the interstitial water. They move through 

 the capillary spaces by wormlike wriggling movements aided by 

 natatory movements of the second antennae, mandibles, and to some 

 extent the first and second maxillae. Most of the body segments- 

 are provided with a pair of lateral, dorsoventral, shallow troughs 

 which have heavily chitinized edges. The indistinct genital aperture 

 is on the first thoracic segment, and the anus is on the last abdominal 

 segment between the bases of the caudal rami. Several larval stages 

 have been found. Only one species is known. 



So far as is known, the Mystacocarida are confined to the intertidal 

 zones of marine beaches. They are a part of the complex community 

 of bacteria, protozoa, and microscopic metazoa including harpacticoid 

 copepods, Tardigrada, Nematoda, Oligochaeta, Acarina, and several 

 other groups which inhabit the capillary water between the grains 

 of sand. The Mystacocarida are probably facultative anaerobes since 

 they may occur in portions of the intertidal sand where oxygen may 

 be either absent or present in small quantities. 



Superficially, the Copepoda and the Mystacocarida are somewhat 

 similar. The two groups may be most easily distinguished from each 

 other, however, on the basis of the number of segments comprising the 

 head, thorax, and abdomen. In copepods the maxilliped segment is 

 fused with five other anterior segments to form the head, and, in 

 addition, the first thoracic segment is usually fused with the head, 

 the whole thus constituting a cephalothorax. In the Mystacocarida, 

 on the other hand, both the maxilliped segment and the first thoracic 

 segment are distinct and separate from the head. Furthermore, the 

 maxilliped segment is clearly separated from the thorax by a ventral 

 constriction. According to the terminology of Sars (1901, 1903-1911) 

 as clarified by Monk (1941), copepods typically have five thoracic 

 (metasome) segments and five abdominal (urosome) segments. Al- 

 though there are 10 segments posterior to the maxilliped segment in 

 the Mystacocarida, there is no specialized movable articulation between 



