4 



The body of tlie American as of the Moluccan King-crab {Limulus) consists of three 

 principal parts — two large, broad, depressed, and shield-shaped, as viewed from above 

 (PI. V A., b), the third long and spike-shaped {ib. c). 



For the description of the external characters of these parts, which are not here 

 noticed, I refer to the elementary works on Crustacea and to the undercited excellent 

 treatise by Van der Hoeven*. 



The homologies propounded by the Dutch monographer have not, however, been 

 generally accepted. " It is evident," he remarks, " that the foremost division [' premier 

 bouclier '] answers to the head and thorax of insects ; for the feet are attached thereto, 

 whilst it bears on its upper surface the organs of vision. Thus the head is here con- 

 founded with the thorax, and we believe ourselves authorized to give to this first buckler 

 the name of ' Cephalothorax,' which naturalists assign to the first part of the body of 

 Arachnidans" f. 



This homology seems not to have been so evident to subsequent crustaceologists. 

 Milne-Edwards, Thos. Bell, Spence Bate, Prof. Dana, and, above all, those eminent 

 observers, Salter, Huxley, "Woodward, who have devoted themselves so laboriously and 

 successfully to the study of the paliBozoic Crustacea, to which Limulus is most closely 

 allied, reject it. According to them, the ' cej)halothorax,' V. der H., answers only to 

 the ' head ' of Insects and Crustaceans. 



There is, of course, a corresponding discrepancy as to the homology of the second 

 division of the body of Limtikis. " Le second bouclier r^pond a I'abdomen des Arach- 

 nides," according to V. der Hoeven J. It is the 'thorax' of the above-cited later 

 carcinologists. 



At this point I venture to submit the following remarks : — The first division (a in all 

 the plates), which constitutes, in Limulus, the major part of the entire body, M^hich 

 includes, besides the mouth, the brain, and organs of sense, also the major part of the 

 neural axis, the same proportion of the heart and of the genital organs, together with the 

 stomach, liver, and half of the intestinal canal, has obvious analogies with both head and 

 abdomen of higher animals. The second division (b in all the plates), which in both 

 Limulus and Scorpio includes the lamellate respu'atory organs, the continuation of the 

 heart, of the intestine, and of the neural axis, with the terminal outlets of the genital 

 organs, as obviously repeats characters of both thorax and abdomen of higher animals. 



The so-termed ' cephalothorax ' of Arachnology, which is, as Van der Hoeven rightly 

 recognized, the homologue of the first division of the body of Limulus, does not include 

 the segments and appendages answering to those called ' thoracic ' in modern crustace- 

 ology. The ' abdomen ' of Scorpio (Audouin) and of Limulus (Van der Hoeven) does 

 correspond with the so-called ' thorax ' of carcinologists. 



To apply the terms ' cephalon,' ' caput,' or ' head,' to the division of the body of Li- 

 mulus, above characterized, seems, however, to be an extension of the use of such term 

 beyond fair and reasonable bounds. 



* Van der Hoeven, ' Ecchcrches sur I'Histoire Naturelle et rAnatomic des Limules,' fol. 1838. The species which 

 he dissected was the rajjier-taUed Molucca Crab {Limulus rotundicauda, Latr.). 

 t Ih. p. 10. t Op. cit. p. 11. 



