TYPE CRUSTACEA. 423 



ment more than the Annelid larva and the comparison will not hold. If 

 the direct Annelid origin is to be accepted, it seems most satisfactory at 

 present to regard the Nauplius as a secondarily acquired larval stage 

 without any ancestral significance. 



Another suggestion has, however, been made which gives the Nauplius 

 a significance and traces the Crustacea back to unsegmented ancestors. It 

 is to the effect that the Nauplius can be referred to Rotiferlike ancestors, 

 the remarkable Hexarthra with its six processes being supposed to indi- 

 cate the line of descent. It is exceedingly doubtful, however, whether 

 this similarity can be regarded as anything more than a superficial one 



TYPE CRUSTACEA. 



I. Class ENTOMOSTRACA. Number of segments varies ; abdomen without 

 appendages ; larva a Nauplius. 



1. Order Phyllopoda. Number of segments variable ; appendages 



with branchiae. 



1. Suborder Branchiopoda. Body plainly segmented and seg- 

 ments of thorax more numerous than six. Apus, Branchipus, 

 Estheria, Limnadia, Limnet-is. 



2. Suborder Cladocem. Body indistinctly segmented ; with bi- 

 valved shell ; four to six thoracic appendages. Daphnia, 

 Moina, Sida, Evadne, Polyphemus. 



2. Order Ostracoda. With bivalved shell ; body indistinctly seg- 



mented ; two thoracic appendages. Cypris, Cythere, Cypri- 

 dina, Halocypris. 



3. Order Copepoda. Without shell ; five pairs of thoracic limbs ; 



many forms parasitic and degenerate. 



1. Suborder Encopepoda. First thoracic segment only fused with 

 head ; abdomen cylindrical and segmented except in highly 

 degenerated forms. Cyclops, Canthocamptus, Harpacticus, 

 Calanus, Cetochilus (free-swimming) ; Notodelphys (commen- 

 salistic) ; Corycceus, Sapp/iirina, Eryasilus, Caligus, Panda- 

 rus (partly parasitic) ; Philichthys, Penella, Lerncea, Chon- 

 dracantnus, Ac7itheres, Anchorella (parasitic). 



2. Suborder Branchiura. All thoracic segments fused with head ; 

 abdomen small and lamellar, partly parasitic. Argulus. 



4. Order Cirrhipedia. Sessile or parasitic ; segmentation indis- 



tinct ; six pairs of thoracic appendages ; pass through Cypris 

 stage. Lepas, Scalpellum, Ibla, Balanus (sessile) ; Alcippe, 

 Cryptophialus (boring) ; Proteolepas, Sacculina, Laura, 

 Dendrogast&r (parasitic) . 



II. Class MALACOSTRACA. Number of segments constant ; thoracic seg- 

 ments eight, abdominal seven or eight. 



1. Subclass Leptostraca. With bivalved shell; abdomen with eight 

 segments. Nebalia. 



