TRANSACTIONS OF SECTION D. 629 



tbe representatives of a group of primitive Malacostraca, through which, by 

 structural divergence, the establishment of the higher crustacean sub-orders may- 

 have come about. 



It is pertinent to this to note that work upon cave-dwelling and terrestrial 

 forms, upon ' well-shrimps ' and the like, has produced important results. And 

 interesting indeed is the recent discovery of three species, living at 800-900 feet 

 above sea-level, in Gippsland, one an amphipod, two ot them isopods, which, though 

 surface-dwellers, are all blind. ^'^ While they prove to be species of genera normally 

 «yed, they in their characters agree with well-known American forms ; and the 

 bleaching of their bodies and atrophy of their eyes proclaim them the descendants 

 •of cave-dwelling or subterranean ancestors, among whom the atrophy took place. 



Huxley in 1880 rationalised our treatment of the higher Crustacea, by 

 devising a classification by gills, expressive of the relationships of these to the 

 limb-bases, interarticular membranes, and body-wall. ^''^ Hardly had his intiuence 

 taken effect when, by work extending over the years 1886 to 1893, in the study 

 of Penaeus, the Pbyllopods, Ostracods, and other forms, evidence had been 

 accumulating to show that the crustacean appendage, even to the mandible itself, 

 lias primarily a basal constituent (protopodite) of three ."egments ; that the 

 branchire one and all are originally appendicular in origin ; and that the numerical 

 reduction of the basal (protopoditic) segments to two, with the assumption of a 

 iion-appeudicular relationship by the gills, is due to coalescence of parts, with or 

 without suppression."' The evidence for this epoch-making conclusion, which 

 simplifies our conceptions and brings contradictory data into line, is as irresistible 

 as it is important, and there has been nothing finer in tbe whole history of 

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

 characters of the anteunule by appeal to embryology goes to the wall ; and, taking 

 a deep breath, we view the Crustacea in a new light. 



There remains for brief consideration one carcinological discovery second to 

 none which bear on the significance of larval forms. It is that of the Trilobite 

 Triarthrus becki, obtained 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 third is certainly and the 

 second is apparently biramous, the first uniramous and antennifcrm. In this we 

 have a combination of characters known only in the nauplius larva among all 

 living crustacean forms ; and the conclusion that the adult trilobite, like the 

 Euphausiacea, Sergestidas, Penaeidae, the Ostracods, and Cirripedes of to-day, was 

 derived by direct expansion of the nauplius larva can hardly be doubted. Much 

 yet remains to be done with the study of the Triarthrus limbs ; and the suggestion 

 of a foliaceous condition by those of the pygidium, which are the youngest, is a 

 remarkable fact, the meaning of which the tuture 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 cannot regard the foliation of their appendages as 

 anything but a specialisation. Be this as it may, the structural community 

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

 in the yolk-bearing higher Crustacean types (e.g., Astucvs) a perceptible halt in the 

 ■development may be observed at the three-limb-bearing stage ; that in Mysis the 

 vitelline membrane is shed but to make way for a nauplius cuticle ; '-^ and that the 

 median nauplius eye has long been found sessile on the adult brain of represen- 

 tative members of the higher crustacean groups, up to the lobster itself','-' our 

 belief in the ancestral significance of the nauplius larval form is established 

 beyond doubt. 



The thought of the nauplius suggests ether larval forms. The gastrula is no 

 longer accepted without reserve; the claims of the blastuln, planula, parenchy- 

 mella, not to say the plakula, have all to be borne in mind."'-- It is of the 

 Trochophore, however, as familiar as the nauplius, that I would rather speak, as 

 influenced by recent research. It is supposed to be primitive for the molluscs and 

 chaetopod worms at least ; and various attempts have been made to bolster it up, 



