598 MORPHOLOGICAL METHOD AND RECENT PROGRESS IN ZOOLOGY. 



crustacean morphology. With it, the attempt to explain the supposed 

 anomalous characters of the anteniuil(> by appeal to embryology goes 

 to the wall; and. taking- a deep bi-eath, we view the Crustacea in a 

 new light. 



There remains for l)rief consideration one carcinological discovery 

 second to none which bear on the signiticance of larval foi'ms. It is 

 that of the trilobite TrlartJirns Bevhi^ ol)tained in abundance from 

 the Lower Silurian near New York, with all its limbs preserved. In 

 the simplicity of its segmentation and the biramous condition of its 

 limbs it is primitive to a degree. Chief among its characters are the 

 total absence of jaws in the strict sense of the term, and the fact that 

 of its three anterior pairs of appendages the tliird is certainly and the 

 second is apparently Inramous, the tirst uniramous and antenniform. 

 In this we have a combination of characters known only in the nau- 

 plius larva among all living crustacean forms; and the conclusion that 

 the adult trilobite, like that of the Euphausiacea, Sergestidte Penaiida^, 

 the Ostracods, and Cirripedes of to-day, was derived by direct expan- 

 sion of the nauplius larva can hardly bo doubted. Much 3'et remains 

 to be done with the stud}" of the Triarthrus limbs; and the suggestion 

 of a f oliaceous condition by those of the pygidium, which are the young- 

 est, is a remarkable fact, the meaning of which the future must decide. 

 We should expect the condition to be a provisional one, since while 

 we admit the primitive nature of the phyllopods as an order, we can 

 not regard the foliation of their appendages as anything hwt a special- 

 ization. Be this as it may, the structural community lietween the 

 nauplius larva and the trilobite is now proved; and when we add that 

 in the yolk-bearing higher Crustacean types (e. g., Astacu-s) a percept- 

 ible halt in the development may be observed at the three-limb- bearing- 

 stage; that in Ift/s/s the vitelline membrane is shed but to make wa}^ 

 for a nauplius cuticle; and that the median nauplius eye has long ])een 

 found sessile on the adult brain of representative members of the 

 higher crustacean groups, up to the lobster itself, our belief in the 

 aricestral significance of the nauplius larval form is established l)eyond 

 doubt. 



The thought of the nauplius suggests other larval forms. The gas- 

 trula is no longer accepted without reserve; the claims of the ])histula, 

 planula, parenchymella, not to say the plakula, have all to be borne 

 in mind. It is of the Trochophore, however, as familiar as the nau- 

 plius, that I would rather speak, as influenced by recent research. It 

 is supposed to be primitive for the molluscs and cliietopod worms at 

 least; and various attempts have been made to ])olstei' it up, and to 

 show that if we allow for adaptive^ change, its chai'acters, well known, 

 are constant within the limits of its simpler forms. 



It is now more than forty years ago that the late Lacaze-Duthiers 

 described for Doiiidnnn a larv:d stage, characterized by the j)()sses- 



