This is shewn in the following table compiled from Claus’ observations.
| Length of Larva. | Appendages of thoracic region; viz. the 2nd and 3rd maxilliped and 5 ambulatory appendages. | Appendages of abdomen. |
|---|---|---|
| 3-3½ mm. | 2nd maxilliped, rudimentary. | 1st abdominal appendage. |
| 3½-4 mm. | 2nd maxilliped, biramous. 3rd rudimentary. 1st and 2nd ambulatory appendages, rudimentary. | 2nd and 3rd abdominal appendages. 4th and 5th rudimentary. |
| 4½-5 mm. | 3rd maxilliped, biramous. | 4th, 5th, and 6th fully developed. |
| 5-5½ mm. | 3rd and 4th ambulatory appendages. | |
| 6 mm. | 5th ambulatory appendage. |
All the appendages following the second pair of maxillæ are biramous, and the first eight of these bear branched gills as their epipodites. It is remarkable that the epipodite is developed on all the appendages anteriorly in point of time to the outer ramus (exopodite).
Although in Mysis there is no free larval stage, and the development takes place in a maternal incubatory pouch, yet a stage may be detected which clearly corresponds with the Nauplius stage of Euphausia (E. van Beneden, No. [465]). At this stage, in which only the three Nauplius appendages are developed, the Mysis embryo is hatched. An ecdysis takes place, but the Nauplius skin is not completely thrown off, and remains as an envelope surrounding the larva during its later development.
Decapoda. Amongst the Decapoda the larva usually leaves the egg in the Zoæa form, but a remarkable exception to this general rule is afforded by the case of one or more species of Penæus. Fritz Müller was the first to shew that the larva of these forms leaves the egg as a typical Nauplius, and it is probable that in the successive larval stages of these forms the ancestral history of the Decapoda is most fully preserved[188].
The youngest known larva of Penæus ([fig. 214]) has a somewhat oval unsegmented body. There spring from it the three typical pairs of Nauplius appendages. The first is uniramous, the second and third are biramous, and both of them adapted for swimming, and the third of them (mandibles) is without a trace of the future blade. The body has no carapace, and bears anteriorly a single median simple eye. Posteriorly it is produced into two bristles.
Fig. 214. Nauplius stage of Penæus. (After Fritz Müller.)
After the first moult the larva has a rudiment of a forked tail, while a dorsal fold of skin indicates the commencement of the cephalo-thoracic shield. A large provisional helmet-shaped upper lip like that in Phyllopods has also appeared. Behind the appendages already formed there are stump-like rudiments of the four succeeding pairs (two pairs of maxillæ and two pairs of maxillipeds); and in a slightly older larva the formation of the mandibular blade has commenced, together with the atrophy of the palp or Nauplius appendage.