ZOOLOGICAL POSITION OF LIMULUS. 



227 



II. 



Trilobita (fossil extinct). 

 I. Eyes sessile, compound. 

 (2. No ocelli.) 



3. Hypostome well developed. 



4. (Appendages partly oral, partly ambulatory or 



natatory, arranged in pairs.) 1 



5. Thoracic segments, variable in number from six 



even to twenty-six, free and movable (animal 

 sometimes rolling in a ball). 

 C. Abdominal somites coalesced, forming broad 

 caudal shield (bearing the branchiae beneath). 



Isopoda (fossil and living). 

 1. Eyes sessile, compound. 

 (2. No ocelli.) 



3. Hypostome small. 



4. Appendages partly oral, partly ambulatory or 



natatory, arranged in pairs. 



5. Thoracic segments, usually seven, free and 



movable (animal sometimes rolling in a ball). 



6". Abdominal somites coalesced, forming broad 

 caudal shield, bearing the branchiae beneath. 



A few words are necessary in explanation as to some points of dissidence between 

 the conclusions arrived at by my friend Dr. A. S. Packard, jr., and myself. 



Dr. Packard evidently relies mainly on embryology in deciding the zoological position 

 which an animal shall occupy, a line of reasoning which I have felt it my duty on a 

 former occasion most earnestly to deprecate. 



Admitting, as I do, most thoroughly the great value of such researches, I nevertheless 

 hold that it is essential to a correct interpretation of an organism to take the sum of all 

 the characters which the species presents, giving due weight to each 3 — a method of 

 investigation in which all our most sound and philosophical naturalists and physiologists 

 are agreed. 



1st. As to the comparison between larval Limulus and the Trilobita, I would observe 

 that before Limulus leaves the egg — nay, even when the embryo is little more than a 

 primitive band or disk traced out on the yolk (see woodcut, fig. 77 and PL XXXIII, 

 fig. 1), it has six pairs of appendages. Dr. Packard states, " I might say that I have 

 examined hundreds (I believe over a thousand) living eggs, as well as those that had 

 been hardened in spirits, in order to ascertain whether the germ first appears with less 

 than six pairs of legs, but without success " (op. cit., p. 200). 



At a little later stage Dr. Packard says (p. 165) "there are six well-marked 

 cep/ialol/ioracic [cephalic, H. W.] segments ; the nine abdominal [thoracico-abdominal, 

 H. W.] rings have appeared, no more being added during after life." 



These facts appear to me to dispose entirely of the attempts to homologise the larva 

 of Limulus with the free-swimming Nauplius of the Copepoda on the one hand, or with 

 the young of the Trilobita on the other. 



The number of paired appendages concealed beneath the double shield of the 

 infant Agnostus, Sao, or Trinucleus, we are entirely unable to discover, it cannot there- 



1 Mr. C. D. Walcott's researches on the appendages of the Trilobites are not yet completed. 



2 'Quart. Journ. Geol. Soc.,' 18/1, vol. xxviii, p. 59. 



