ON OUR PRESENT KNOWLEDGE OF THE CRUSTACEA. 201 



.The mandibles are without an appendage. The oral limbs are assum- 

 ing much of their permanent form, except the tetartognatbus, -which is 

 not visible. The gnathopoda are developed very similarly to those in the 

 Brachyura zoaea, but exhibit a joint more in the development of the 

 primary branch of the second pair. One solitary hair, differing in length, 

 structure, and position from the others, appears to be constant in all forms 

 of the Anomura zoaea. In some species it is nearer the base of the apical 

 joint than in others ; but it is invariably constant, extremely long, and 

 furnished with very long and delicate cilia, that are inserted at right angles 

 with the main stalk of the hair. I do not remember having observed it 

 on any zoaea but those of the Anomurous Crustacea. 



The zosea of the Porcellanidae (PI. VI., fig. 2) may readily be distin- 

 guished from those of Pagurus by the length of the rostrum and postero- 

 lateral processes of the carapace, which sometimes equal the length of the 

 animal, and sometimes half; by having unarmed spinal processes to 

 represent the second antenna?, instead of a ciliated squamose branch, and 

 by having the last somite of the pleon terminating in a broad flat plate, 

 the posterior margin of which is posteriorly produced to a point, instead 

 of being hollowed, while it carries five ciliated hairs on each side of the 

 median line. In one species, Claus figures the termination as produced 

 to a long spine. 



The zoaea of Galathea(P\.V., fig. 6)may be distinguished by the posterior 

 margin of the carapace being definitely serrated ; by two dorsal teeth on the 

 posterior margin of the somites of the pleon ; by the extremely long ovate 

 eye occupying about one half the length of the carapace ; by the presence of 

 a sharp serrated tooth at the distal extremity of the basal joint of the 

 second antennae ; by the shortness of the cylindrical branch that ter- 

 minates in a small tooth and one ciliated hair ; and the sharply pointed 

 distal angle of the squamose branch of the same antennae. 



The zoaea of Dromia (PI. VI., fig. 1), much resembles that of Pagurus, 

 except that it appears to have no posterior processes at the infero-distal 

 angles of the carapace ; but more especially by the form of the telson, which 

 is extremely deeply cleft in the median line of the posterior margin. 



The brephalus of the Mafiroura differs veiy much in the character in 

 which it quits the ovum in separate genera. Those that leave it in the 

 zoaea (PI. V., fig. 5) condition are very distinguishable from those of the 

 Brachyura and Anomura. 



The zoaea of the common Shrimp (Crangon vulgaris) may be taken 

 as the type of the form. It differs from the zoaea of the common Prawn 

 (Palcemon squilla) in very small details, one of which is in having a pointed 

 rostrum that it loses with the second moult. 



The carapace, long, narrow and moderately compressed, furnished with 

 a slender rostrum projecting horizontally forwards, having no projection 

 or tooth along the posterior and lateral margins. 



The pleon consists of six somites, of which the last is expanded into a 

 flat membranous plate, slightly indented in the median line of the pos- 

 terior margin, and fringed with six ciliated hairs, and one, the external, 

 small spine-like point. 



The eyes, a, a, are large, and obliquely ovate. 



The first antennae, b, b, are two-jointed ; the basal joint is long and cylin- 

 drical ; it supports at the outer distal angle a stiff ciliated spine, and at the 

 extremity the second branch, which appears in the form of a small uni- 

 articulate joint, out of which near the extremity another seems ready to 



