270 CRUSTACEA. 



FIRST SUB-CLASS. ENTOMOSTRACA. 

 These are the more primitive Crustaceans, often small 

 and simple, with a variable number of segments and append- 

 ages. The newly hatched larva is usually a Nauplius. The 

 adult may retain the unpaired simple frontal eye, which is 

 always found in the Nauplius, and has no gastric mill. 

 Order i. Phyllopoda. 

 Order 2. Ostracoda. 

 Order 3. Copepoda. 

 Order 4. Cirripedia. 



Order I. Phyllopoda. In these at least four pairs of swimming feet 

 bear respiratory plates. The body is generally well segmented, 

 and is protected by a shield-like or bivalve shell. The mandibles 

 are without palps, and the maxillse are rudimentary. 

 (a) Branchiopoda. The body has numerous segments and (10-20 

 or more) appendages with respiratory plates. The shell is 

 rarely absent, usually shield-like or bivalved. The heart is a 

 long dorsal vessel with numerous openings. The eggs are able 

 to survive prolonged desiccation in the mud. 



Branchipus, a beautifully coloured fresh water form, with 



hardly any shell. 



Artemia. Brine shrimps. Periodically parthenogenetic. 

 By gradually changing the salinity of the water, 

 Schmankewitsch was able, in the course of several 

 generations, to modify A. salina into A. milhlenhausii^ 

 and vice versa. Artemia fertilis is one of the four 

 animals known to occur in the dense waters of Salt Lake. 

 Limnadia^ with bivalve shell. Periodically parthenogenetic. 

 A mollusc-like bivalve shell is still more marked in 

 Estheria. 



Apus, a fresh water form with a large dorsal shield. 

 Periodically parthenogenetic. One species herma- 

 phrodite. 



Of these Apus is certainly the most interesting. It is over an inch in 

 length, and therefore a giant among Entomostraca. It has an 

 almost world wide distribution. "It possesses peculiarities of 

 organisation which mark it out as an archaic form, probably 

 standing nearer to the extinct ancestors of the Crustacea than 

 almost any other living member of the group." The appendages 

 are very numerous and mostly leaf-like. They may be regarded 

 as representing a primitive type of Crustacean limb. Professor 

 Ray Lankester enumerates them as follows : 



fi. Antenna. 

 Pre-oral. -! 2. Second antenna. (This is sometimes absent, and 



apparently always in certain species. ) 

 ^3. Mandible. 

 Oral. -[ 4. Maxilla. 



( 5. Maxillipede. 



