REPORT ON THE CRUSTACEA MACRURA. vii 



MORPHOLOGY. 



Milne-Edwards laid it down in his earlier writings that the type of the Decapod 

 Crustacea consists of twenty-one somites, of which the anterior seven belong to the 

 cephalon or head, the posterior seven to the pleon or abdomen, and the intermediate 

 seven to the pereion or thorax. 



Dana admits that there are normally twenty-one segments, and twenty-one corre- 

 sponding pairs of appendages, the posterior seven of which belong to the pleon. But he 

 says that of the remaining fourteen pairs, only five are subservient of locomotion, the 

 other nine being organs of special sense or in relation to manducation and placed about 

 the mouth. In reaching this conclusion, Dana was guided by the results of his examina- 

 tion of the Brachyura and higher Macrura, in which the nervous system is most highly 

 centralised. 



From the study of development as well as of the adult structure of the more simple 

 forms of Crustacea, I previously adopted and maintained the view put forward by Milne- 

 Edwards. But since then, from the examination of extensive series of Crustacea of all 

 groups and types, and of many forms in different stages of development, I have been 

 led to reconsider this conception of the structural relationship of the several parts. 



If we turn to the development of the Synaxidea we find some of the most instructive 

 examples of crustacean form. In this group the animal leaves the egg far advanced 

 beyond the Zoea stage, and exists in what Anton Dohrn calls the Megalopa stage ; 

 although in character it is far below the form to which Leach originally gave that name, 

 and which was ultimately shown to be an advanced stage of a young Brachyura. It 

 is extremely thin and very translucent, and a more advanced form has been named 

 Phyllosoma by Milne-Edwards. At the period when it is hatched it is about 2 mm. 

 in length (PI. XIIa. figs. 1, 2), and is distinctly divided into three separate parts. The 

 anterior portion or cephalon is broad and shield-like, and represents the future carapace 

 of the adult ; the second portion or pereion is also broad and disc-like, and it was upon 

 the characters of these two divisions that a supposed family was established by Milne- 

 Edwards under the name of Bicuirasse's ; the third portion or pleon is a narrow terminal 

 process. 



The cephalon consists of the ocular, the two antennal, the mandibular, and the first 

 post-oral somite (PL XIIb. fig. 1 ; PI. XIIc. fig. 2). The two anterior somites, as shown 

 in the adult animal, are separate from those which form the large dorsal shield or 

 carapace. Studying the development of the Phyllosoma still further in various species, 

 we find that the succeeding somites are distinct from the cephalon and together compose 

 the pereion ; consecpiently the whole of the appendages attached to this division must be 



