10 A. S. PACKAED, JE., ON THE ANATOMY 



Cirripedia and Copepoda, where what ultimately become antennae and mandibles are 

 swimming feet ; and in the zoea of Decapods, in which two pairs of antennae exist, and 

 the temporary swimming feet ultimately become maxillae and maxillipedes ; (2) the 

 unique relations of the inferior blood system to the central nervous system (the brain 

 and certain nerves alone excepted) ; and (3) the peculiar nature of the eyes of the Meros- 

 tomata and Trilobites, which are constituted on a type peculiar to themselves. 



Under all these circumstances, it may be claimed, as has been done by A. Milne- 

 Edwards, that the Merostomata should form a distinct class of Arthropoda. It should 

 be borne in mind, however, that M. A. Milne-Edwards believes that a second class 

 of Arthropods should be formed to receive the Trilobites. Taking all the facts into 

 consideration, we should propose that the Merostomata and Trilobites should together 

 form a subclass of Crustacea {i. e., Branchiate Arthropods) standing parallel to, and 

 as the equivalents of, all the other Crustacea, the two groups being parallel and 

 equally important branches of the same genealogical tree. 



It should be borne in mind that the Palaeocarida are a generalized or synthetic type ; 

 Limulus is, so to speak, a subzoea, the cephalothorax having been differentiated from the 

 abdomen and prematurely developed, with the gills of a normal crustacean ; having the 

 primitive appendages of a nauplius, and the compound eyes superficially like those of 

 a zoea, but on an elementary, prematurely developed type ; while the circulatory system 

 is of a high order, and the nervous system well developed, though the brain is constituted 

 on a simple plan, quite unlike that of the higher Crustacea, and probably the Crustacea 

 in general. The subclass of Palaeocarida apparently bears very much the same relation 

 to the subclass Neocarida, as the subclass Elasmobranchii or Ganoidea do to the 

 Teleostean fishes ; as in these early synthetic forms certain organs are prematurely 

 developed, while the skeleton and other parts are in a more or less embryonic or larval 

 condition. They abounded most in the Palaeozoic ages, dying out in part, with 

 but a few survivors ; such was the case with the Palaeocarida. Under these circumstances 

 we see no more reason for removing the Merostomata and Trilobita from the class 

 of Crustacea, than to consider the Elasmobranchii or Ganoids as independent classes 

 of Vertebrates, or the Arachnids or Myriopoda, as independent classes of Arthopoda. 



Regarding, then, the Palaeocarida as an early offshoot of the Crustacean or Branchiate 

 Arthropod tree or stem, we would venture to present the classification on the following 

 page, as proposed in 1879, in our little school book, " Zoology." 



The Neocarida may be characterized briefly as genuine Crustacea with two pairs of 

 antennae, biting mouth-parts and ambulatory or swimming thoracic feet ; mostly modern 

 types. The Palaeocarida, on the other hand, have the cephalothoracic appendages in 

 the form of foot-jaws, rather than true jaws ; no antennae, the brain supplying the 

 compound eyes and ocelli alone ; the nerves to the cephalothoracic appendages sent off 

 from an oesophageal ring or collar ; and the nervous system, with the exception of the 

 brain, ensheathed in a ventral system of arteries ; they are mostly palaeozoic types. 



The close homologies between the Merostomata and Trilobita were discussed in our 

 first memoir. At that time (p. 184), we advocated the view that the cephalothoracic 

 limbs of the Trilobites must have been jointed, rounded rather than foliaceous, and ambu- 

 latory in function, and inclined to the views of Mr. Billings as to the nature of what he 



