CRUSTACEA. 503 



would seem to be the following. The segments with their appendages of 

 Arthropoda and Annelida are normally formed from before backwards, 

 therefore every member of these two groups with more than three segments 

 must necessarily pass through a stage with only three segments, and the fact 

 that in a particular group this stage is often reached on the larva being 

 hatched is in itself no proof that the ancestor of the group had only three 

 segments with their appendages. This explanation appears to me, so far 

 as it goes, quite valid ; but though it relieves us from the necessity of 

 supposing that the primitive Crustacea had only three pairs of appendages, 

 it does not explain several other peculiarities of the Nauplius 1 . The more 

 important of these are the following. 



1. That the mandibles have the form of biramous swimming feet and 

 are not provided with a cutting blade. 



2. That the second pair of antennae are biramous swimming feet with a 

 hook used in mastication, and are innervated (?) from the subcesophageal 

 ganglion. 



3. The absence of segmentation in the Nauplius body. An absence 

 which is the more striking in that before the Nauplius stage is fully reached 

 the body of the embryo is frequently divided into three segments, e.g. 

 Copepoda and Cirripedia 



4. The absence of a heart. 



5. The presence of a median single eye as the sole organ of vision. 



Of these points the first, second, and fifth appear only to be capable of 

 being explained phylogenetically, while with reference to the absence of a 

 heart it appears very improbable that the ancestral Crustacea were without 

 a central organ of circulation. If the above positions are accepted the 

 conclusion would seem to follow that in a certain sense the Nauplius is 

 an ancestral form but that, while it no doubt had its three anterior pairs 

 of appendages similar to those of existing Nauplii, it may perhaps have 

 been provided with a segmented body behind provided with simple biramous 

 appendages. A heart and cephalo-thoracic shield may also have been 

 present, though the existence of the latter is perhaps doubtful. There was 

 no doubt a median single eye, but it is difficult to decide whether or no 

 paired compound eyes were also present. The tail ended in a fork between 

 the prongs 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 immediately 

 before the appendages lose their Nauplius characters (fig. 208 B), or a 

 Cyclops larva just before the Cyclops stage (fig. 229), than the earliest 

 Nauplius of either of these forms. 



If the Nauplius ancestor thus reconstructed is admitted to have existed, 

 the next question in the phylogeny of the Crustacea concerns the relations 

 of the various phyla to the Nauplius. Are the different phyla descended 

 from the Nauplius direct, or have they branched at a later period from 



1 For the characters of Nauplius vide p. 460. 



