Behind these appendages there are moreover formed the rudiments of six pairs of feet. Under the cuticle of the first pair of antennæ there may be seen just before the final moult the four-jointed antennæ of the Cypris stage with the rudiment of a disc on the second joint by which the larvæ eventually become attached.
By the free Cypris stage, into which the larva next passes, a very complete metamorphosis has been effected. The median and paired eyes are present as before, but the dorsal shield has become a bivalve shell, the two valves of which are united along their dorsal, anterior, and posterior margins. The two valves are further kept in place by an adductor muscle situated close below the mouth. Remains of the lateral horns still persist. The anterior antennæ have undergone the metamorphosis already indicated. They are four-jointed, the two basal joints being long, and the second provided with a suctorial disc, in the centre of which is the opening of the duct of the so-called antennary or cement gland, which is a granular mass lying on the ventral side of the anterior region of the body. The gland arises (Willemoes Suhm) during the Nauplius stage in the large upper lip. The two distal joints of the antennæ are short, and the last of them is provided with olfactory hairs. The great upper lip and second pair of antennæ and mandibles have disappeared, but a small papilla, forming the commencement of the adult mandibles, is perhaps developed in the base of the Nauplius mandibles. The first pair of maxillæ have become small papillæ and the second pair probably remain. The six posterior pairs of appendages have grown out as functional biramous swimming feet, which can project beyond the shell and are used in the locomotion of the larva. They are composed of two basal joints, and two rami with swimming hairs, each two-jointed. These feet resemble Copepod feet, and form the main ground for the views of Claus and others that the Copepoda and Cirripedia are closely related. They are regarded by Claus as representing the five pairs of natatory feet of Copepoda, and the generative appendages of the segment behind these. Between the natatory feet are delicate chitinous lamellæ, in the spaces between which the cirriform feet of the adult become developed. The ventral spinous process of the Nauplius stage is much reduced, though usually three-jointed. It becomes completely aborted after the larva is fixed.
In addition to the antennary gland there is present, near the dorsal side of the body above the natatory feet, a peculiar paired glandular mass, the origin of which has not been clearly made out, but which is perhaps equivalent to the entomostracan shell-gland. It probably supplies the material for the shell in succeeding stages[202].
Fig. 232. Larval forms of the Thoracica. (From Huxley.)
A. Nauplius of Balanus balanoides. (After Sp. Bate.) B. Pupa stage of Lepas australis. (After Darwin.)
n. antennary apodemes; t. cement gland with duct to antenna.
The free Cypris stage is not of long duration; and during it the larva does not take food. It is succeeded by a stage known as the pupa stage ([fig. 232] B), in which the larva becomes fixed, while underneath the larval skin the adult structures are developed. This stage fully deserves its name, since it is a quiescent stage during which no nutriment is taken. The attachment takes place by the sucker of the antennæ, and the cement gland (t) supplies the cementing material for effecting it. A retrogressive metamorphosis of a large number of the organs sets in, while at the same time the formation of new adult structures is proceeded with. The eyes become gradually lost, but the Nauplius eye is retained, though in a rudimentary state, and the terminal joints of the antennæ with their olfactory hairs are thrown off. The bivalve shell is moulted about the same time as the eyes, the skin below it remaining as the mantle. The caudal process becomes aborted. Underneath the natatory feet, and between the above-mentioned chitinous lamellæ, the cirriform feet are formed; and on their completion the natatory feet become thrown off and replaced by the permanent feet. In the Lepadidæ, in which the metamorphosis of the pupa stages has been most fully studied, the anterior part of the body with the antennæ gradually grows out into an elongated stalk, into which pass the ovaries, which are formed during the Cypris stage. At the base of the stalk is the protuberant mouth, the appendages of which soon attain a higher development than in the Cypris stage. At the front part of it a large upper lip becomes formed. Above the mantle and between it and the shell there are formed in the Lepadidæ the provisional valves of the shell. These valves are chitinous, and have a fenestrated structure, owing to the chitin being deposited round the margin of the separate epidermis (hypodermis) cells. These valves in the Lepadidæ “prefigure in shape, size, and direction of growth, the shelly valves to be formed under and around them” (Darwin, No. [519], p. 129).
Whatever may be the number of valves in the adult the provisional valves never exceed five, viz. the two scuta, the two terga and the carina. They are relatively far smaller than the permanent valves and are therefore separated by considerable membranous intervals. They are often preserved for a long time on the permanent calcareous valves. In the Balanidæ the embryonic valves are membranous and do not overlap, but do not present the peculiar fenestrated structure of the primordial valves of the Lepadidæ.
In connection with the moult of the pupa skin, and the conversion of the pupa into the adult form, a remarkable change in the position takes place. The pupa lies with the ventral side parallel to and adjoining the surface of attachment, while the long axis of the body of the young Cirriped is placed nearly at right angles to the surface of attachment. This change is connected with the ecdyses of the antennary apodemes (n), which leave a deep bay on the ventral surface behind the peduncle. The chitinous skin of the Cirriped passes round the head of this bay, but on the moult of the pupa skin taking place becomes stretched out, owing to the posterior part of the larva bending dorsalwards. It is this flexure which causes the change in the position of the larva.