REPORT ON THE CRUSTACEA MACRURA. 217 



Division DENDROBRANCHIATA. 



This Division forms a natural and well-defined series of families that differ from the 

 Trichobranchiata and Phyllobranchiata in anatomical structure, external form, develop- 

 ment, and general habits, and may be divided into a Normal and an Aberrant group. 



The branchial structure, instead of being developed in the form seen in the Normal 

 group of the other two divisions, consists of a series of plumes, that in their more typical 

 condition are attached by, or very near, their basal extremity to the animal, and from a 

 lono; central stalk send off on each side a single row of branches that divide and 

 subdivide in a more or less distinctly different manner in separate genera, and sometimes 

 also in otherwise well-defined species. 



In some genera this distinction is so great that in Philonicus the branchial petals on 

 a casual inspection may be mistaken for those of a phyllobranchiate Macruran, but even 

 here they differ in form on the same plume, being more decidedly typical of the Dendro- 

 branchiata at the base, but falling off in character as they approach the apex. 



In the Aberrantia the branchial plumes are fewer in number and more elementary in 

 structure, but still characteristic of the division. 



The nervous system exists as a series of separate ganglionic centres, corresponding to 

 every somite posterior to the cephalic system, with the exception of that of the posterior 

 somite of the pereion, which is absent, the nerve which supplies the fifth or ultimate pair 

 of pereiopoda being derived from the same ganglion as that which supplies the preceding 

 pair. Examination of several genera induces me to believe that this is a very constant 

 character throughout the division. 



Corresponding to the depreciated condition of their nerve centre, the posterior two 

 pairs of pereiopoda are invariably of an enfeebled character as compared with the three 

 preceding pair's, and they are never chelate in either sex during any period of the exist- 

 ence of the animal. 



In the Trichobranchiata, as well as in the Phyllobranchiata, the ovum when liberated 

 from the oviduct is connected with the pleopoda by a filamentous attachment, and is 

 thus suspended until such time as the embryo is sufficiently advanced to be hatched, 

 and to swim freely in the ocean. The brephalos appears either in the Zoea, Phyllosoma, 

 or Megalopa stage. 



In the Dendrobranchiata the form of the brephalos is unknown, except in Lucifer. 

 In 1863 Fritz Midler observed great numbers of the young of Crustacea, of various 

 forms, swimming about in the sea during the summer months. These he examined, 

 and he was able to establish a chain of progressive forms that induced him to believe he 

 had discovered the development of some prawn, which he supposed to be Penseus. 



The first great fink, the determination of the parent of the earliest form, was wanting, 

 and it is remarkable that though more than twenty years have passed, not one of the 



(ZOOL. CHALL. EXP. PART LII. 1886.) Fff 28 



