HOMOLOGIES OF THE CRUSTACEAN LIMB. 



411 



as is so ^ell marked in the maxillipedes, and the thoracic le^'S possess 

 not even the rudiments. Change of function and radical changes of 

 structure are most extreme in the 

 Malacostracous Crustacea, from the 

 Brachyura to the Isopoda and Am- 

 phipoda. If so startling in these com- 

 paratively recent forms, it is not to 

 \)e wondered at that still greater and 

 more fundamental modifications of 

 the Crustacean type obtain in the 

 archaic forms, the Pahtsocarides, of 

 which Limulus is the sole survivor. ^ 

 To those who insist on the Arachni- 

 dau affinities of the Merostomata, we s 

 would suggest that the same shifting 

 and change of function and structure 

 is to be observed among the Trache- 

 ate Arthropoda, and that Limulus is 

 not less a genuine Branchiate Ar- 

 thropod for presenting some features 

 analogous to the Arachnida. 



A study of the Phyllopoda and cp 

 Phyllocarida must tend to confirm 

 the view we have expressed as to the 

 synthetic, or generalized nature of 

 Limuhis, while we have in another 

 place endeavored to show in the 

 light of A. Milne-Edwards' anatom- 

 ical studies on Limulus, that it is 

 an abnormal Crustacean and far re- 

 moved from the Branchiopoda; there 

 are nevertheless some points in which 

 it comes in contact with the Phyllo- 

 poda, and which have been noticed 

 ever since the time when O. ¥. Muller 

 comprised Apus in his genus ^^ Limu- 

 lus.'" If the reader will comj^are the 

 accompanying longitudinal section of 

 Limulus with our section of A])us 

 in PI. XXXVII, some striking resem- 

 blances will be seen 5 external ij^ the 

 front edge of the carapace, i. e., the 

 frontal doublure, so well adapted for 

 burrowing in the mud ; the relations 

 of the hypostoma or labrum, and the 

 retention of the ocelli, as well as the 

 mode of moulting the shell, are ex- 

 ternal points of resemblance, while riG.il.-SectionthroiighasmillLinmluapol.y 

 . . 4j-, ., „ , J r- ,-, 1 - pnenius (much enlai-cjeU), to compare with a ueo 



internally the iront part Ot the head candousCiustaceau'such as Apus: Iw. llvor; pr, 



filled with the lobules of the liver,f^^'^^ ^i^^^^U^^^^;,^;^^ 



the oblique, long, narrow, ceSOpha-intBstiue; «, anus; Ir, brain; m, moiuh; or, 



gus,thepositionofthestomach under '^'^"^^'^""'^"^ '^^"»' ''^' ^Mommai ganglia. 

 the eyes so far in front in the head, the siini3le archi-cerebrum, the gen- 

 eral form of the heart, and the guathob.ases near the mouth are addi- 

 tional points of resemblance. 



