418 GEOLOGICAL SURVEY OF THE TEERITORIES. 



Hence the three orders of BrancJiiopoda, the Phyllocarida, and the Bee- 

 apoda (with the Tetrad ecapod a) must, it appears to us, have indepen- 

 dently of each other originated from some Laurentian Nauplius-like 

 form. 



The views of Claus and some important criticisms upon them are 

 given at length by Mr. Balfour in his valuable Comparative Embryol- 

 ogy, while we would observe that neither Claus, Dohrn, nor Balfour 

 appear to refer to the palteontological history of the Crustacea. 



Professor Claus, in his suggestive work on the genealogy of Crustacea, 

 according to Balfour, claims that the later Nauplius stages of the dif- 

 ferent Entomostracan groups and the Malacostraca {Penceus larva) ex- 

 hibit undoubted Phyllopod affinities. He therefore postulates the earlier 

 existence of a Protophyllopod form, from which he believes all the Crus- 

 tacean groups to have diverged. This ancestral form, Balfour thinks, 

 had three anterior j)airs of appendages similar to those of existing 

 Nauplii. It may have had a segmented body behind the third pair of 

 appendages provided with simple biramous appendages. A heart and 

 cephalothoracic shield may also have been present, though the existence 

 of the latter is perhaps doubtful. There was no doubt a median simple 

 eye, but, adds Balfour, it is difficult to decide whether or no paired com- 

 pound eyes were also present. The tail ended in a fork, between the 

 X3rongs of which the anus opened, and the mouth was protected by a 

 large upper lip. " In fact, it may very probably turn out that the most 

 primitive Crustacea more resembled an Apus larva at the moult imme- 

 diately before the appendages lose their JSTauplius characters (fig. 208 

 B), or a Cyclops larva just before the Cyclops stage (fig. 229), than the 

 earliest Nauplius of either of these forms" (Balfour, j). 418). 



That the Decapods and Phyllopods may have originated from such a 

 form as Balfour thus depicts seems to us to be quite probable. 



Mr. Balfour, on page 880, states that " the BrcmcMopoda, comprising 

 under that term the Phyllopoda and Cladocera, contain the Crustacea with 

 the maximum number of segments and the least differentiation of the 

 separate appendages. This and other considerations render it probable 

 that they are to be regarded as the most central group of the Crusta- 

 ceans, and as in many respects least modified from the ancestral type 

 from which all the groups have originated." 



Against this view may be, however, offered two criticisms. The ex- 

 cessive number of segments in the Apodidm is paralleled among the 

 Tracheafa by the Chilopods, in which the numerous segments appear to 

 have each two pairs of feet, and these Myriopoda are j)robably not the 

 more ancestral, generalized Myriopodous forms, the Pauropus and 

 Eurypauropus being much more so, these forms having few segments, 

 'each with no more than one pair of feet. The excessive number of seg- 

 ments in A2ms and the irrelative repetition of abdominal feet appear to 

 us to be signs of a vegetative repetition of parts in a type which has 

 •culminated, and is subject to decline and extinction. 



Again, iu Polyartemia, with its 19 pair of feet, where Artemia has the 

 normal number of eleven (within its family limits) appears to us like 

 Apus to be a highly specialized and extreme form } Artemia being the 

 more generalized, though, as compared with Branchiptis, a degrada- 

 tional form. 



The view that the Phyllopods "are members of a group which was 

 previously much larger and the most central of all the Crustacean 

 groups," is not fully sustained by zoogeography or palseoutology. At 

 present all Phyllopods are fresh- water forms, and they are exceptionally 

 Tare forms, occurring only locally, though every continent has its quota 



