The larva undergoes a number of successive ecdyses, at each of which the body becomes more elongated, and certain other changes take place. First of all a pair of appendages arises behind the mandibles, which form the maxillæ ([fig. 229] B); at the same time the basal joint of the maxillæ develops a cutting blade. Three successive pairs of appendages ([fig. 229] C) next become formed—the so-called maxillipeds (the homologues of the second pair of maxillæ), and the two first thoracic limbs. Each of these though very rudimentary is nevertheless bifid. The body becomes greatly elongated, and the caudal fork more developed.
Up to this stage of development the Nauplius appendages have retained their primitive character almost unaltered; but after a few more ecdyses a sudden change takes place; a cephalo-thoracic shield becomes fully developed, and the larva comes to resemble in character an adult Copepod, from which it mainly differs in the smaller number of segments and appendages. In the earliest ‘Cyclops’ stage the same number of appendages are present as in the last Nauplius stage. There ([fig. 229] D) is a well-developed cephalo-thorax, and four free segments behind it. To the cephalo-thoracic region the antennæ, mandibles, maxillæ, the now double pair of maxillipeds (derived from the original single pair of appendages), and first pair of thoracic appendages (p1) are attached. The second pair of thoracic appendages (p2) is fixed to the first free segment, and the rudiment of a third pair (p3) projects from the second free segment. The first pair of antennæ has grown longer by the addition of new joints, and continues to increase in length in the following ecdyses till it attains its full adult development, and then forms the chief organ of locomotion. The second pair of antennæ is much reduced and has lost one of its rami. The two rami of the mandibles are reduced to a simple palp, while the blade has assumed its full importance. The maxillæ and following appendages have greatly increased in size. They are all biramous, though the two rami are not as yet jointed. The adult state is gradually attained after a number of successive ecdyses, at which new segments and appendages are added, while new joints are formed for those already present.
Parasita. The earliest developmental stages of the parasitic types of Copepoda closely resemble those of the free forms, but, as might be expected from the peculiarly modified forms of the adult, they present a large number of secondary characters. So far as is known a more or less modified Nauplius larva is usually preserved.
Fig. 230. Successive stages in the development of Achtheres percarum.
(Copied from Bronn; after Claus.)
A. Modified Nauplius stage. B. Cyclops stage. C. Late stage of male embryo. D. Sexually mature female. E. Sexually mature male.
at1. first pair of antennæ; at2. second pair of antennæ; md. mandible; mx. maxillæ; pm1. outer pair of maxillipeds; pm2. inner pair of maxillipeds; p1. first pair of legs; p2. second pair of legs; z. frontal organ; i. intestine; o. larval eye; b. glandular body; t. organ of touch; ov. ovary; f. rod projecting from coalesced maxillipeds; g. cement gland; rs. receptaculum seminis; n. nervous system; te. testis; v. vas deferens.
The development of Achtheres percarum, one of the Lernæopoda parasitic in the mouth, etc. of the common Perch, may be selected to illustrate the mode of development of these forms. The larva leaves the egg as a much simplified Nauplius ([fig. 230] A). It has an oval body with only the two anterior pairs of Nauplius appendages; both of them in the rudimentary condition of unjointed rods. The usual median eye is present, and there is also found a peculiar sternal papilla, on which opens a spiral canal filled with a glutinous material, which is probably derived from a gland which disappears on the completion of the duct. The probable function of this organ is to assist at a later period in the attachment of the parasite to its host. Underneath the Nauplius skin a number of appendages are visible, which become functional after the first ecdysis. This takes place within a few hours after the hatching of the Nauplius, and the larva then passes from this rudimentary Nauplius stage into a stage corresponding with the Cyclops stage of the free forms ([fig. 230] B). In the Cyclops stage the larva has an elongated body with a large cephalo-thoracic shield, and four free posterior segments, the last of which bears a forked tail.
There are now present eight pairs of appendages, viz. antennæ (two pairs), mandibles, maxillæ, maxillipeds, and three pairs of swimming feet. The Nauplius appendages are greatly modified. The first pair of antennæ is three-jointed, and the second biramous. The outer ramus is the longest, and bears a claw-like bristle at its extremity. This pair of appendages is used by the larva for fixing itself. The mandibles are small and connected with the proboscidiform mouth; and the single pair of maxillæ is small and palped. The maxillipeds (pm1 and pm2) are believed by Claus to be primitively a single biramous appendage, but early appear as two distinct structures[197], the outer and larger of which becomes the main organ by which the larva is fixed. Both are at this stage simple two-jointed appendages. The two anterior pairs of swimming feet have the typical structure, and consist of a protopodite bearing an unjointed exopodite and endopodite. The first pair is attached to the cephalo-thorax and the second (p2) to the first free thoracic segment. The third pair is very small and attached to the second free segment. The mouth is situated at the end of a kind of proboscis formed by prolongations of the upper and lower lips. The alimentary tract is fairly simple, and the anus opens between the caudal forks.
Between this and the next known stage it is quite possible that one or more may intervene. However this may be the larva in the next stage observed ([fig. 230] C) has already become parasitic in the mouth of the Perch, and has acquired an elongated vermiform aspect. The body is divided into two sections, an anterior unsegmented, and a posterior formed of five segments, of which the foremost is the first thoracic segment which in the earlier stage was fused with the cephalo-thorax. The tail bears a rudimentary fork between the prongs of which the anus opens. The swimming feet have disappeared, so also has the eye and the spiral duct of the embryonic frontal organ. The outer of the two divisions of the maxilliped have undergone the most important modification, in that they have become united at their ends, where they form an organ from which an elongated rod (f) projects, and attaches the larva to the mouth or gills of its host. The antennæ and jaws have nearly acquired their adult form. The nervous system consists of supra- and infra-œsophageal ganglia and two lateral trunks given off from the latter. At this stage the males and females can already be distinguished, not only by certain differences in the rudimentary generative organs, but also by the fact that the outer branch of the maxillipeds is much longer in the female than in the male, and projects beyond the head.