The peculiar larval form of the Loricata (Scyllarus, Palinurus) has long been known under the name Phyllosoma ([fig. 222] C), but its true nature was first shewn by Couch (No. [474]) [Couch did not however recognise the identity of his larva with Phyllosoma; this was first done by Gerstäcker] and shortly afterwards by Gerbe and Coste. These observations were however for a long time not generally accepted, till Dohrn (No. [477]) published his valuable memoir giving an account of how he succeeded in actually rearing Phyllosoma from the eggs of Scyllarus and Palinurus, and shewing that some of the most remarkable features of the metamorphosis of the Loricata occur before the larva is hatched.
The embryo of Scyllarus in the egg first of all passes through the usual Nauplius stage, and then after the formation of a cuticle develops an elongated thoracico-abdominal region bent completely over the anterior part of the body. There appear moreover a number of appendages and the rudiments of various organs; and the embryo passes into a form which may be described as the embryonic Phyllosoma stage. In this stage there are present on the anterior part of the body, in front of the ventral flexure, two pairs of antennæ, mandibles, two pairs of maxillæ, the second commencing to be biramous, and a small stump representing the first pair of maxillipeds. The part of the body bent over consists of a small quadrate caudal plate, and an appendage-bearing region to which are attached anteriorly three pairs of biramous appendages—the second and third maxillipeds, and the anterior pair of ambulatory legs—and two pairs of undivided appendages—the second and third pairs of ambulatory legs. In a slightly later stage the first pair of maxillæ becomes biramous, as also does the first pair of maxillipeds in a very rudimentary fashion. The second and third pairs of ambulatory legs become biramous, while the second and third maxilliped nearly completely lose their outer ramus. Very small rudiments of the two hinder ambulatory legs become formed. If the embryo is taken at this stage (vide [fig. 222] A, which represents a nearly similar larva of Palinurus) out of the egg, it is seen to consist of (1) an anterior enlargement with a vaulted dorsal shield enclosing the yolk, two stalked eyes, and a median eye; (2) a thoracic region in which the indications of segmentation are visible with the two posterior pairs of maxillipeds (mxp2 and mxp3) and the ambulatory legs (p1); (3) an abdominal region distinctly divided into segments and ending in a fork.
Fig. 222. Larvæ of the Loricata. (After Claus.)
A. Embryo of Palinurus shortly before hatching.
B. Young Phyllosoma larva of Scyllarus, without the first maxilliped, the two last thoracic appendages, or the abdominal appendages.
C. Fully-grown Phyllosoma with all the Decapod appendages.
at1. antenna of first pair; at2. antenna of second pair; md. mandible; mx1. first maxilla; mx2. second maxilla; mxp1-mxp3. maxillipeds; p1-p3. thoracic appendages.
Before the embryo becomes hatched the first pair of maxillipeds becomes reduced in size and finally vanishes. The second pair of maxillæ becomes reduced to simple stumps with a few bristles, the second pair of antennæ also appears to undergo a retrogressive change, while the two last thoracic segments cease to be distinguishable. It thus appears that during embryonic life the second pair of antennæ, the second pair of maxillæ, and the second and third pair of maxillipeds and the two hinder ambulatory appendages undergo retrogressive changes, while the first pair of maxillipeds is completely obliterated!
The general form of the larva when hatched ([fig. 222] B) is not very different from that which it has during the later stages within the egg. The body is divided into three regions: (1) an anterior cephalic, (2) a middle thoracic, and (3) a small posterior abdominal portion; and all of them are characterised by their extreme dorso-ventral compression, so that the whole animal has the form of a three-lobed disc, the strange appearance of which is much increased by its glass-like transparency.
The cephalic portion is oval and projects slightly behind so as to overlap the thorax. Its upper surface constitutes the dorsal shield, from which there spring anteriorly the two compound eyes on long stalks, between which is a median Nauplius eye. The mouth is situated about the middle of the under surface of the anterior disc. It leads into a stomach from which an anterior and a lateral hepatic diverticulum springs out on each side. The former remains as a simple diverticulum through larval life, but the latter becomes an extremely complicated glandular structure.