6 A. S. PACKARD, JR., ON THE ANATOMY 



prolongation of the ventral appendages on the adjacent part of the abdomen, and by 

 several other organic characters. They are distinguished from all other articulated 

 animals by the disposition of their circulatory system, and consequently, in spite of 

 the small number of species of this group, the zoologist should consider them as 

 constituting a particular class intermediate between the Crustacea and Arachnida. He 

 claims with Mr. H. Woodward, that the fossil Pterygoti and Eurypteri should be united 

 with the Limuli, under the name of Merostomata. Milne-Edwards then adds that " the 

 Merostomata were contemporaries of the Trilobites, and there seems to be between 

 these two groups, not only very strong resemblances, but intermediate forms which establish 

 the passage from one to the other. Some authors have thought it useful to unite them 

 under a common name. This seems to me at least too premature, because we know 

 nothing of significance on the subject of the appendicular system of Trilobites, and we 

 cannot pronounce legitimately on this question ; but it should be taken into consideration, 

 that it seems very probable that the Trilobites differ from the Crustacea properly so-called, 

 as we have seen the Merostomata differ from them, and that they should likewise 

 constitute a particular class in the great natural division of Entomozoa." 



In November, 1873,^ in the light of A. Milne-Edwards' researches, I stated that "I 

 should no longer feel warranted in associating Limulus and the Merostomata generally 

 with the Branchiopoda, but regard them as perhaps forming with the Trilobites a distinct 

 sub-class of Crustacea. In a second notice in the same Journal for December, 1879, I 

 proposed the name Palaeocarida, for the sub-class ; these comprising the Merostomata and 

 Trilobites. We also proposed the term Neocarida for the remaining sub-class of normal 

 Crustacea. 



In 1874 Gegenbaur, in his Grundriss der Vergl. Anatomic, divides the living Branchiata 

 as opposed to the Arthropoda Tracheata, in two divisions : I. Crustacea, II. Poecilopoda.^ 



As regards the relations of the Merostomata to the Arachnida let us examine them and 

 inquire whether they are not rather those of analogy, than of affinity. It is not neces- 

 sary, in view of what has been published, for us to restate the essential anatomical charac- 

 teristics of Limulus. The relations of the viscera to the body wall, and of the appen- 

 dages may be seen by our figures in Plates I and II. 



The resemblances to the Arachnida in general, and the scorpion in particular, have been 

 supposed to consist (1) in the want of antennae, and (2) the form of the central nervous 

 system, as well as (3) the mode of development, while (4) the branchiae of Limulus 

 have been homologized with the pulmonary sacs of spiders. 



It should be borne in mind, however, that the Arachnida are a sub-class of Tracheata, 

 with no antennae to be sure, but with two pairs of post-oral appendages, the mandibles 

 and maxillae, which are constructed on the hexapodous type, and are also built upon the 

 same plan of structure as the mouth-appendages of Myriopoda ; so close indeed are the 

 homologies between the Hexapoda, or insects pi-oper, and the Arachnida and Mj^riopoda, 



> Farther ohsiTvations on the embryolo},')' of Limulus, Palaeocarida, and regard Gegenbaur's Crustacea as equiv- 



with notes on its affinities. Amer. Naturalist, Novcni- aleut to my Neocarida; this would express my views as to 



ber, 1873. the relations of the two sub-classes. This makes the terms 



' If we substitute for the term Poecilopoda, which applies Criixlacea and Branchiala synonyms from my point of 



only to the sub-order of which Limulus is the type, the term view. 



