412 GEOLOGICAL SUEVEY OF THE TEERITORIES. 



In Ms little tract on Worms and Crustacea,^ Professor Hyatt refers 

 to the simple eyes of Limiilus, as if they were the primitive eyes, re- 

 tained from larval life. The structure of the two simple eyes of Limulus 

 appears to he in some important respects quite different from that of 

 Apus, Estheria, and other Phyllopods, in which there is a circle of 

 cones, while in Limulus there is a single large corneal lens on the same 

 plan as the facets of the composed eye of the same animal. If, how- 

 ever, these simple eyes, be regarded as sur\4vors of the primitive larval 

 eye, it would suggest that Limulus and all Merostomata which have 

 similar eyes, have like the Neocarida, descended from a Nauplius an- 

 cestor; although the development of Limulus polyphemus has been 

 shown to be an abbreviated one, the young hatching in the form of the 

 adult. The presence of the single eyes would of course be an argu- 

 ment for its Crustacean affinities ; while on the other hand the posses- 

 sion of compound eyes is a still more important Crustacean character. 



Another point of interest is the mode of moulting in Limulus as com- 

 pared with Apus. From our childhood we have found the cast shells 

 of Limulus, with the carapace split around the edge of the doablure, 

 and we have a i)artially moulted specimen in alcohol. We have not 

 seen a cast skin of Apus, but on asking Dr. Gissler, who has raised the 

 young Apus from the egg^ as to the mode of exuviation in this Crus- 

 tacean, he writes me as follows : " I am certain that the larvae of Apus 

 (from skins examined) split across or just in front of the eyes, and with 

 two or three jerks the animal rids itself of the underlying skin." It 

 would appear then that Apus, which is shaped in front so much like 

 Limulus moults in a nearly similar manner. 



In a general way we accept the homologies pointed out by Professor 

 Lankester between the Phyllopodous leg and the maxillae and maxilli- 

 pedes of the cray^fish, but think that he, in common with Professor 

 Huxley, pushes the homologies too far when he proceeds (on p. 365) to 

 compare minutely the first leg of Apus with the third maxillipedes of 

 Astacus. We do not, as we have stated on p. 391, regard the axis of 

 Apus as truly jointed, and he stretches his homologies entirely too far 

 when he attempts to homologize the first and second endites of Apus 

 with the coxopodite of Astacus; and the third and fourth exites of 

 Apus with the basipodite of Astacus. We would suggest that here, as 

 among the orders of Arachnida, or Hexapoda, or Myriopoda^ if we do not 

 stop at a certain point, we are led into erroneous and misleading attempts 

 at too close homologies. We should, it seems to us, bear in mind the 

 fact that there are ordinal and class homologies; or, in other words, 

 there are different degrees of blood relationship, i. e., different and more 

 or less parallel branches of the Crustacean genealogical tree. 



The Decapods did not descend directly from the Phyllopods, but by 

 a longer line, independent on the one hand from the Phyllocaridous an- 

 cestral line, and on the other from the Branchiopodous stem or branch. 

 But a comparison between the Phyllopodous leg and Decapod maxillae 

 and maxillipedes shows that the Decapod exopodite is but a modified 

 endopodital lobe, and is not homologous with the exites of the Ph\ 1- 

 lopods, the latter corresponding to the epipodite (or gills and flabellum, 

 of the Decapods. We have seen that in all Phyllopods the gill and fla- 

 bellum are differentiated iDarts of the epipodal portion of the leg (epipo- 

 dite). Huxley's view, that the base of the corm or " protopodite" of the 

 first thoracic foot is the endopodite, and the endites are merely secoud- 



iBoston Society of Natural History. Guides for Science Teaching, No. VII, Worms 

 and. Crustacea. By Alpheus Hyatt, Boston, 1882. 



