GENERAL OBSERVATIONS. 



The classification of the Macrura into three separate divisions, according to the 

 structural character of one of their most important and essential organs, will be found 

 convenient in the study of this order in relation to its distribution both in time and 

 space. 



Each division is apparently of equal importance both in relation to size and structure, 

 but however much they may correspond in certain anatomical details which are character- 

 istic of the separate divisions, or may approximate to one another in general form, yet 

 they are essentially distinct in affinity, both in their development and descent. Inde- 

 pendent of the character of the branchiae, the Trichobranchiata, Dendrobranchiata, and 

 Phyllobranchiata are separated generally from each other by the form of the carapace, 

 the appendages of the pereion, the plan of the rhipidura, and the stage at which the 

 embryo quits the ovum. 



In the Dendrobranchiata the brephalos is supposed to exist in the Nauplius condition, 

 from positive evidence in the genus Lucifer ; and from negative evidence of all the other 

 genera this is supposed to obtain universally throughout the division. 



In the Phyllobranchiata the brephalos, so far as it has been observed, is a Zoea, 

 excepting in the case of a few genera in which closely allied forms vary, such as Alpheus 

 and Somaralpheus, Acanihephyra and Syxtellasp>is, and Crangon, in which it appears 

 that the only separation beyond specific character is that of this variation in the stage of 

 the brephalos. 



In the Trichobranchiata the brephalos is in the Megalopa condition, of which 

 Phyllosoma is an immature stage ; this is universal in the division, excepting in the 

 genus Stenopus. According to their branchial structure both Stenopus and Spongicola 

 belong to the Trichobranchiata, while the appearance of their carapace resembles that 

 of the Phyllobranchiata, and the structure of their legs corresponds with that 

 of the Dendrobranchiata ; whereas in the manner of their development, the 

 brephalos of Stenopus appears to be a Megalopa, approaching in form that of the 

 Trichobranchiata, while that of Spongicola is a Zoea, approaching that of the 

 Phyllobranchiata. 



