CRUSTACEA. 479 



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 antennae, the second pair of maxillae, 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: (i) 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. 



At the front border of the disc is placed the unjointed but elongated 

 first pair of antennae (/'). Externally to and behind these there spring the 

 short posterior antennas (at'*}. At the base of which the green gland is 

 already formed. Surrounding the mouth are the mandibles (md) and anterior 

 pair of maxillae (mx l ), and some distance behind the second pair of maxillae 

 (tnx\ consisting of a cylindrical basal joint and short terminal joint armed 

 with bristles. The first pair of maxillipeds is absent. 



The thoracic region is formed of an oval segmented disc attached to the 

 under surface of the cephalic disc. From its front segment arises the second 

 pair of maxillipeds (tnxp*) as single five-jointed appendages, and from the 

 next segment springs the five-jointed elongated but uniramous third pair 

 of maxillipeds (mxjp), and behind this there arise three pairs of six-jointed 

 ambulatory appendages (p l , p~, p 3 , of which only the basal joint is represented 

 in the figure) with an exopodite springing from their second joint. The two 

 posterior thoracic rings and their appendages cannot be made out. 



The abdomen is reduced to a short imperfectly segmented stump, ending 

 in a fork, between the prongs of which the anus opens. Even the youngest 

 larval Phyllosoma, such as has just been described, cannot be compared with a 

 Zoasa, but belongs rather, in the possession of biramous thoracic feet, to a 

 Mysis stage. In the forked tail and Nauplius eye there appear however to 

 be certain very primitive characters carried on to this stage. 



The passage of this young larva to the fully formed Phyllosoma (fig. 

 222 C) is very simple. It consists essentially in the fresh development of 

 the first pair of maxillipeds and the two last ambulatory appendages, the 

 growth and segmentation of the abdomen, and the sprouting on it of biramous 



