THE MALACOSTRACA. ^63 



The Cirripedia are almost exclusively marine, only a few 

 species tolerating even brackish water. The Thoracica alone 

 have yet been found in the fossil state. The oldest known 

 genus, Poflicipes, occurs in the lower oolite ; there is a single 

 cretaceous species of Verruca, but the sessile Cirripedes be- 

 come numerous only in the tertiary epoch. 



The Rhizocephala (Peltogaster, Sacculina) are small and 

 parasitic ; usually upon the abdomen of other Crustacea 

 (Podophtficdmia). The body is like a sac or disk, and devoid 

 of segmentation and of limbs. The aperture of the sac is 

 funnel-shaped, and supported by a ring of chitin. The circum- 

 ference of the funnel gives off a number of root-like processes, 

 which branch out through the body of the infested animal. 

 The alimentary canal is obsolete, and there are no cement- 

 glands. They are hermaphrodite, and the young, like those 

 of the other Pectostraca, pass through a Nauplius and a 

 Cypris stage. 1 



The Malacostraca. — The groups of Crustacea known as 

 the Podophthalmia, the Cumacea, the Edriophthalmia, and 

 the Stomatopoda, are here included under this head. 



The body consists of twenty somites (counting that which 

 bears the eyes as one), and, of these, six (bearing the eyes, 

 antennules, antennas, mandibles, and two pairs of maxillae) 

 constitute the head ; eight enter into the thorax, and bear 

 the foot-jaws and ambulatory limbs ; and six form the abdo- 

 men and swimming limbs. In some few instances the num- 

 ber of somites is reduced, but they never exceed twenty. 



The J\ T auplius4orm of the free embryo is rare, but occurs 

 in some cases (Penevs). In others (Mysis) it is represented 

 only by a temporary condition of the embryo, during which, 

 however, a chitinous cuticula is formed, and subsequently 

 shed ; and what appear to be remains of such a transitory 

 record of an original Nauplms state, are seen in many A?n- 

 phipoda and Isopoda, which nearly attain their adult form 

 within the egg. In most Podophthalmia the embryo leaves 

 the egg not as a Nauplius, but as a Zoaza, which has thora- 

 cic, but no abdominal, appendages, and in many respects re- 

 sembles a Copepod. 



1 The term Ci/pris-sta.ge, usually applied to that condition of the larvae of 

 the Pectostraca in which they are provided with a bivalve carapace, must not 

 be taken to imply any special affinity with the Ostracoda. On the contrary, 

 the larva in the Cypris-stage is much more similar to a Copepod or Branchi- 

 opod. 



