188 STUDIES IN EVOLUTION 



indicated in the descriptions and observations of the protaspis 

 and its derived characters, and need not be repeated here. 

 Apparently the nauplius presents nothing homologous, unless 

 possibly the frontal sensory organs of Apus, Balanus, Peneus, 

 etc., may be taken as such. The paired eyes and frontal 

 sensory organs are close together and seem to have some inti- 

 mate connection, for, as the paired eyes develop, the latter 

 dwindle and disappeear. Likewise in the trilobites the free- 

 cheeks bear the visual areas, and may be almost wholly con- 

 verted into eyes as in ^iEglina (Qydopyge). 



The greater or less separation of the cerebral ganglia in the 

 chaetopods and in some of the lower Crustacea leads to the 

 idea that the free-cheeks in trilobites are the pleura of an 

 occuliferous head segment, which otherwise is lost. If the 

 hypostoma is homologous with the annelid prostomium, as 

 urged by Bernard, 11 then the free-cheeks may be considered 

 as representing the second procephalic segment, which is the 

 number required on the supposition that each neuromere cor- 

 responds to a somite. There is a separate neuromere to each 

 mesodermic metamere posterior to the head, and from anal- 

 ogy we should expect that each neuromere in the cephalon 

 would represent an original segment, especially as it can be 

 demonstrated that the head is composed of consolidated or 

 fused segments (Kingsley 24 ). 



Having thus shown the probable ventral structure of the 

 protaspis, we are prepared to make some general observations 

 on the larval type of modern Crustacea known as the nau- 

 plius. Before doing this it is well to emphasize again that 

 there is very positive evidence, amounting virtually to cer- 

 tainty, that the protaspis had five pairs of limbs attached to 

 the cephalic portion, behind which was an abdominal portion 

 containing the formative elements out of which all the pos- 

 terior somites and appendages were developed. 



The Crustacean Nauplius. 



The name Nauplius was first used by O. F. Miiller 29 to 

 designate a minute crustacean believed to represent an adult 



