176 



A. S. PACKARD, JR., ON THE 



same as in Ptorygotus. The line between Mr. Woodward's thorax and abdomen of the 

 Eurypteridse seems to me to be a purely imaginary one. So also in Limulus has Mr. Wood- 

 ward drawn an imaginary line across the abdomen. (See Quart. Journ. Geol. Soc, London, 

 Feb., 186.', pi. H., fig. 1.) But that I may not seem to do injustice to his views, I quote 

 his account of the homologies of Limulus below.^ 



So fai', then, as the external form of this region of the body is concerned, with its ap- 

 pendages, it forms in Limulus a true head, consisting of six segments, each bearing a pair 

 of limbs, the antennary segment on the tergal surface a pair of simple eyes, and the third 

 or mandibular segment a pair of compound eyes. On examining the arrangement of the 

 viscera, however, we find that this head contains organs that in other Branchiopoda 

 and in the Malacostraca ^ are lodged in the thoracic region. We may consider the thorax 

 in Limulus as potential, so far as regards the presence of external segments, but real when 

 we regard the disposition of the internal organs, so that we may feel justified in applying 

 the convenient term cephalothorax to the anterior division of the body, just as we woidd 

 call the same region in the zoea a cephalothorax, before the thoracic segments are devel- 

 oped. By a reference to fig. 27, on plate v, we see that this cephalothorax contains the 

 stomach, and a considerable portion of the intestine, and the liver, which opens into the 

 intestine near the middle of the cephalothorax, sending but a single pair of biliary tubes 



* " In Limulus the head is composed of a broad buckler, 

 having the larval and compound eyes upon its superior or 

 convex surface, and the mouth, surrounded by six pairs of 

 chelate appendages, placed beneath the head-shield. The 

 first pair of appendages is placed in front of, and the four 

 succeeding paii-s are posterior to, the mouth. These latter 

 are furnished with spinous gnathites, the most posterior pair 

 serving as maxillce. Next follows the operculum, or thoracic 

 plate, which is attached to the posterior margin of the head, 

 and bears upon its inner and upper surface the reproductive 



organs or ovaries The anterior portion here 



spoken of as the head has always been regarded as repre- 

 senting the cephalothor.ix, and the posterior portion as the 

 abdomen." [Savigny and Milne Eilwards, (though in Todd's 

 Cyclopsedia, art. Crustacea, p. 772, Expl. fig. 411, he calls the 

 base of the abdomen thorax,) and Van derHoeven, together 

 with several other authors, down to the present time, have 

 regarded the anterior division of the body as the head, and 

 the posterior as the abdomen. On the other hand, Latreille, 

 in his last work in the second edition of the Ecgne Animal, 

 says, in a foot note, vol. i, 1829, p. 187: ''Les deux pieds an- 

 terieurs pourraient representer les mandibules des decapodes; 

 les quatre suivants, leurs machoires, et les dix derniers, leurs 

 pieds-machoires; ceux du second bouclier repondraient aux 

 pieds thoraciques"; while Professor Dana's views (Crustacea, 

 Exp. Exp., 18.52) as to the homologies of Limulus, are as 

 follows: "In Limulus the body consists of three segments, 

 and they may be compared to the segments in Caligus. The 

 anterior segment bears six pairs of members ; the ^7-s( appears 

 to correspond to the second pair of antennie (or third nor- 

 mal segment), the second, (Jiird, fourth,Jifth and sixth, to the 

 mandibles and the four following pairs of members (or the 

 fourth to the eighth normal segments inclusive). In the 

 Caligus the last pair here referred to is natatory, and the 

 carapax is divided just anterior to it, instead of posterior. 



The second segment of the body, which we consider a? a 

 continuation of the cephalothorax, and not abdominal, bears 

 six pairs of foliaceous organs, analogous to the foliaceous ap- 

 pendages of the posterior part of the thorax in certain Cali- 

 gid.e. in some of which one or two pairs of legs are com- 

 bined into a hard, thin plate, like an apron. These six pairs 

 make up exactly the normal number of cephalothorax seg- 

 ments, — fourieen The abdomen, according to 



this view, is confined to the last or third segment." He re- 

 marks in another place, Amer. Journ. Sc., vol. xxxvi, Nov. 

 1863, p. 34G, "The joint across the carapax of the Limulus 

 corresponds in position to a suture or imperfect articulation 

 in the carapax of the Caligi, etc."] "Tliis latter," continues 

 Mr. Woodward, "I propose to call the thorax (in part), and 



to give my reasons for doing so as we proceed 



If the head in Limulus be composed of the cephalothorax, 

 and the posterior portion be the abdomen, then it follows 

 that the opercular plate is thoracic, and the succeeding 

 branchial plates are abdominal, thus difi'ering widely from 

 the fossil forms, in which the head is simply the head, with 

 one thoracic segment added to it, bearing the opercular plate; 

 the branchiae are thoracic, and the abdomen is entirely desti- 

 tute of appendages. Let it be granted, however, that the 

 head in Limulus represents the entire cephalothorax, and 

 the posterior portion the abdomen; if specimens can be 

 shown having a 'post-abdominal' series of segments, I think it 

 will be conceded by all carcinologists that in Limulus the 

 abdomen is rudimentary, as is the case in Cyclops, Daphnia, 

 Lerna»a, etc., and in the Brachyurous Decapods." 



' The terms Malacostraca and Entomostraca we only use 

 for convenience, believing them to bo usefiil but artificial 

 categories ; as we see the Decapoda graduating through the 

 Phyllopodous Nebalia, etc., into the Branchiopoda, and the 

 latter (through the Cladocera) into the Ostracodes, Cope- 

 poda, and more remotely into the Cirripedia. 



