MORPHOLOGY OF CYCLOPS. 39 



groups of the Crustacea except the Arthrostraca *, if not in the adult, at least in the larva, 

 and contemporaneous with or anterior to the paired compound eye. 



The ordinary paired Crustacean eye exists often side hy side with the Nauplius eye, 

 constantly in all the Malacostraca, in most Phyllopoda, in some Ostracoda (Cypridinidse, 

 &c), in the Branchiura (Argulus), in the Cirriped pupa, hut not in the Phizocephala. 



We may infer that the Nauplius eye is primitive — in Copepoda proper it is functional, 

 and hence susceptible of a degree of variation of which it is incapable when superseded and 

 reduced to a mere larval organ. We may also infer that the compound and paired eye of 

 Crustacea appeared later, in a stock which was ancestral to those which have it. But 

 Argulus and the Cirripedia and Rhizocephala have distinct Copepodan affinities. We may 

 conclude, then, that the stock which first acquired this form of the organ, and was ancestral 

 to the rest of the Crustacea, Avas itself a descendant of the Copepoda. 



Again, a recently acquired organ would be less constant than one long possessed ; and 

 we find just in these groups of puzzling affinities the inconstancy we might expect on 

 this hypothesis : Cirripedian pupae have the compound eye, Phizocephala lack it ; in 

 Ostracoda the Cypridinidae alone possess it. This reasoning would lead us to the 

 following phylogeny : — 

 Copepoda. 



Eudiplopida (a hypothetical group) . 



Branchiura. 



Ostracoda. Protophyllopoda. 



Cirripedia. 



Nebaliidae. Phyllopoda. 



Arthrostraca. Thoracostraca. 

 (b & b l ). Condition of appendages. 



1. Antennules. — As mentioned, this pair is always uniramous — the primitive larval 



condition. 



2. Antenna?. — In some groups biramous, in others uniramous by loss of exopodite. 



3. Mandibles. — Possessing in some groups their primitive biramous palp, never more 



than uuiramous in other adult Crustacea. 



4. Maxilla? I. — Most plastic ; showing every transition from the primitive mastica- 



tory blade with a biramous palp to the closest approach to the type of the 



Phyllopod limb, in which all Lankester's divisions, except, perhaps, the " bract," 



may be made out. 



I dwell the more on this appendage, because I think, with Clans f, that the biramous 



condition is primitive, and that if, as is almost certain, the oral thoracic appendages of 



Malacostraca have passed through a Phyllopod stage, that stage is phylogenetically 



secondary to the biramous condition ; while there is no proof that the abdominal limbs 



have been at all Phyllopod-like in ontogeny or phylogeny. On the contrary, those 



* Giard has shown that even in this group the larva of Entoniseus has a Nauplius eye. 



t Crustaceen-System, p. 17. Considering this and other things, I cannot see why he calls the primitive Crustacea 

 " Protophyllopoda." I use the term in a restricted sense, as will be seen from the above phylogeny. 



