The larvæ of Cypridina are hatched in a condition which to all intents and purposes resembles the adult.
Phylogeny of the Crustacea.
The classical work of Fritz Müller (No. [452]) on the phylogeny of the Crustacea has given a great impetus to the study of their larval forms, and the interpretations of these forms which he has offered have been the subject of a very large amount of criticism and discussion. A great step forward in this discussion has been recently made by Claus in his memoir (No. [448]).
The most fundamental question concerns the meaning of the Nauplius. Is the Nauplius the ancestral form of the Crustacea, as is believed by Fritz Müller and Claus, or are its peculiarities and constant occurrence due to some other cause? The most plausible explanation on the second hypothesis 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[203]. 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 antennæ are biramous swimming feet with a hook used in mastication, and are innervated (?) from the subœsophageal 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.