The larval stages commence with a Nauplius[199] which, though regarded by Claus as closely resembling the Copepod Nauplius ([figs. 231] and [232] A), certainly has very marked peculiarities of its own, and in some respects approaches the Phyllopod Nauplius. It is in the youngest stage somewhat triangular in form, and covered on the dorsal side by a very delicate and hardly perceptible dorsal shield, which is prolonged laterally into two very peculiar conical horns ([fig. 231] lh), which are the most characteristic structures of the Cirriped Nauplius. They are connected with a glandular mass, the secretion from which passes out at their apex. Anteriorly the dorsal shield has the same extension as the body, but posteriorly it projects slightly.
An unpaired eye is situated on the ventral surface of the head, and immediately behind it there springs a more or less considerable upper lip (lb), which resembles the Phyllopod labrum rather than that of the Copepoda. Both mouth and anus are present, and the hind end of the body is slightly forked in some forms, but ends in others, e.g. Lepas fascicularis, in an elongated spine. The anterior of the three pairs of Nauplius appendages (At1) is uniramous, and the two posterior (At2, and md) are biramous. From the protopodites of both the latter spring strong hooks like those of the Copepod and Phyllopod Nauplii. In some Nauplii, e.g. that of Balanus, the appendages are at first not jointed, but in other Nauplii, e.g. that of Lepas fascicularis, the jointing is well marked. In Lepas fascicularis the earliest free Nauplius is enveloped in a larval skin, which is thrown off after a few hours. The Nauplii of all the Thoracica undergo a considerable number of moults before their appendages increase in number or segmentation of the body appears. During these moults they grow larger, and the posterior part of the body—the future thoracic and abdominal region—grows relatively in length. There also appear close to the sides of the unpaired eye two conical bodies, which correspond with the frontal sense organs of the Phyllopods. During their growth the different larvæ undergo changes varying greatly in degree.
In Balanus the changes consist for the most part in the full segmentation of the appendages and the growth and distinctness of the dorsal shield, which forms a somewhat blunt triangular plate, broadest in front, with the anterior horns very long, and two short posterior spines. The tail also becomes produced into a long spine.
Fig. 231. Nauplius larva of Lepas fascicularis viewed from the side.
oc. eye; At. 1. antenna of first pair; At. 2. antenna of second pair; md. mandible; lb. labrum; an. anus; me. mesenteron; d.sp. dorsal spine; c.sp. caudal spine; Vp. ventral spine; lh. lateral horns.
In Lepas fascicularis the changes in appearance of the Nauplius, owing to a great spinous development on its shield, are very considerable; and, together with its enormous size, render it a very remarkable form. Dohrn (No. [520]), who was the first to describe it, named it Archizoæa gigas.
The dorsal shield of the Nauplius of Lepas fascicularis ([fig. 231]) becomes somewhat hexagonal, and there springs from the middle of the dorsal surface an enormously long spine (d.sp), like the dorsal spine of a Zoæa. The hind end of the shield is also produced into a long caudal spine (c.sp) between which and the dorsal spine are some feather-like processes. From its edge there spring in addition to the primitive frontal horns three main pairs of horns, one pair anterior, one lateral, and one posterior, and smaller ones in addition. All these processes (with the exception of the dorsal and posterior spines) are hollow and open at their extremities, and like the primitive frontal horns contain the ducts of glands situated under the shield. On the under surface of the larva is situated the unpaired eye (oc) on each side of which spring the two-jointed frontal sense organs. Immediately behind these is the enormous upper lip (lb) which covers the mouth[200]. At the sides of the lip lie the three pairs of Nauplius appendages, which are very characteristic but present no special peculiarities. Posteriorly the body is produced into a long ventral spine-like process (Vp) homologous with that of other more normal Nauplii. At the base of this process large moveable paired spines appear at successive moults, six pairs being eventually formed. These spines give to the region in which they are situated a segmented appearance, and perhaps similar structures have given rise to the appearance of segmentation in Spence Bate’s figures. The anus is situated on the dorsal side of this ventral process, and between it and the caudal spine of the shield above. The fact that the anus occupies this position appears to indicate that the ventral process is homologous with the caudal fork of the Copepoda, on the dorsal side of which the anus so often opens[201].
From the Nauplius condition the larvæ pass at a single moult into an entirely different condition known as the Cypris stage. In preparation for this condition there appear, during the last Nauplius moults, the rudiments of several fresh organs, which are more or less developed in different types. In the first place a compound eye is formed on each side of the median eye. Secondly there appears behind the mandibles a fourth pair of appendages—the first pair of maxillæ—and internal to these a pair of small prominences, which are perhaps equivalent to the second pair of maxillæ, and give rise to the third pair of jaws in the adult (sometimes spoken of as the lower lip).