an1. and an2. First and second antennæ; md. mandible; mx. maxilla; l. labrum; fr. frontal sense organ; f. caudal fork; s. segments.
The term has now been extended to a very large number of larvæ which have certain definite characters in common. They are provided ([fig. 208] A) with three pairs of appendages, the future two pairs of antennæ and mandibles. The first pair of antennæ (an1) is uniramous and mainly sensory in function, the second pair of antennæ (an2) and mandibles (md) are biramous swimming appendages, and the mandibles are without the future cutting blade. The Nauplius mandibles represent in fact the palp. The two posterior appendages are both provided with hook-like prominences on their basal joints, used in mastication. The body in most cases is unsegmented, and bears anteriorly a single median eye. There is a large upper lip, and an alimentary canal formed of œsophagus, stomach and rectum. The anus opens near the hind end of the body. On the dorsal surface small folds of skin frequently represent the commencement of a dorsal shield. One very striking peculiarity of the Nauplius according to Claus and Dohrn is the fact that the second pair of antennæ is innervated from a sub-œsophageal ganglion. A larval form with the above characters occurs with more or less frequency in all the Crustacean groups. In most instances it does not exactly conform to the above type, and the divergences are more considerable in the Phyllopods than in most other groups. Its characters in each case are described in the sequel.
Phyllopoda..For the Phyllopoda the development of Apus cancriformis may conveniently be taken as type (Claus, No. [454]). The embryo at the time it leaves the egg ([fig. 208] A) is somewhat oval in outline, and narrowed posteriorly. There is a slight V-shaped indentation behind, at the apex of which is situated the anus. The body, unlike that of the typical Nauplius, is already divided into two regions, a cephalic and post-cephalic. On the ventral side of the cephalic region there are present the three normal pairs of appendages. Foremost there are the small anterior antennæ (an1), which are simple unjointed rod-like bodies with two moveable hairs at their extremities. They are inserted at the sides of the large upper lip or labrum (l). Behind these are the posterior antennæ, which are enormously developed and serve as the most important larval organs of locomotion. They are biramous, being formed of a basal portion with a strong hook-like bristle projecting from its inner side, an inner unjointed branch with three bristles, and an outer large imperfectly five-jointed branch with five long lateral bristles. The hook-like organ attached to this pair of appendages would seem to imply that it served in some ancestral form as jaws (Claus). This character is apparently universal in the embryos of true Phyllopods, and constantly occurs in the Copepoda, etc.
The third pair of appendages or mandibles (md) is attached close below the upper lip. They are as yet unprovided with cutting blades, and terminate in two short branches, the inner with two and the outer with three bristles.
At the front of the head there is the typical unpaired eye. On the dorsal surface there is already present a rudiment of the cephalic shield, continuous anteriorly with the labrum (l) or upper lip, the extraordinary size of which is characteristic of the larvæ of Phyllopods. The post-cephalic region, which afterwards becomes the thorax and abdomen, contains underneath the skin rudiments of the five anterior thoracic segments and their appendages, and presents in this respect an important variation from the typical Nauplius form. After the first ecdysis the larva ([fig. 208] B) loses its oval form, mainly owing to the elongation of the hinder part of the body and the lateral extension of the cephalic shield, which moreover now completely covers over the head and has begun to grow backwards so as to cover over the thoracic region. At the second ecdysis there appears at its side a rudimentary shell-gland. In the cephalic region two small papillæ (fr) are now present at the front of the head close to the unpaired eye. They are of the nature of sense organs, and may be called the frontal sense papillæ. They have been shewn by Claus to be of some phylogenetic importance. The three pairs of Nauplius appendages have not altered much, but a rudimentary cutting blade has grown out from the basal joint of the mandible. A gland opening at the base of the antennæ is now present, which is probably equivalent to the green gland often present in the Malacostraca. Behind the mandibles a pair of simple processes has appeared, which forms the rudiment of the first pair of maxillæ (mx).
In the thoracic region more segments have been added posteriorly, and the appendages of the three anterior segments are very distinctly formed. The tail is distinctly forked. The heart is formed at the second ecdysis, and then extends to the sixth thoracic segment: the posterior chambers are successively added from before backwards.
At the successive ecdyses which the larva undergoes new segments continue to be formed at the posterior end of the body, and limbs arise on the segments already formed. These limbs probably represent the primitive form of an important type of Crustacean appendage, which is of value for interpreting the parts of the various malacostracan appendages. They consist ([fig. 209]) of a basal portion (protopodite of Huxley) bearing two rami. The basal portion has two projections on the inner side. To the outer side of the basal portion there is attached a dorsally directed branchial sack (br) (epipodite of Huxley). The outer ramus (ex) (exopodite of Huxley) is formed of a single plate with marginal setæ. The inner one (en) (endopodite of Huxley) is four-jointed, and a process similar to those of the basal joint is given off from the inner side of the three proximal joints.
Fig. 209. Typical phyllopod appendage. (Copied from Claus.)