AND EMBRYOLOGY OF LIMULUS. 9 



Turning now to the relations of the Merostomata to the normal Crustacea, we may 

 inquire whether the former belong to the class of Crustacea, or should form the type 

 of a distinct class. The latter view is that proposed by A. Milne-Edwards, and a number 

 of zoologists have adopted this view. 



The facts that seem to us to point to the crustacean nature of Limulus and 

 its allies are: (1) the nature of the branchiae, those of Limulus being developed in 

 numerous plates overlapping each other on the second abdominal limbs ; those of 

 the Eurypterida being, according to H. Woodward, attached side by side, like the 

 teeth of a rake ; while the mode of respiration, as seen on plate 1, is truly 

 crustacean; (2) the resemblance of the cephalothorax of Limulus to that of Apus; 

 (3) the general resemblance of the gnathopods to the feet of the Nauplius 

 or larva of the Cirripedia and Copepoda ; (4) the digestive tract is homologous 

 throughout with that of Crustacea, particularly the Decapoda, there being no urinary 

 tubes as in Tracheata; (5) the heart is on the crustacean type as much as on the tracheate 

 type, and the internal reproductive organs (ovaries and testes) open externally, at the base 

 of and in the limbs, much as in Crustacea. 



The resemblances and differences between the normal Crustacea (Neocarida) and the 

 Palaeocarida (Merostomata and TrUobita) are shown in the following tabular view : — 



COMPAKISON OF NORMAL CRUSTACEA (NeOCAEIDA) WITH LlJIULUS AND OTHER PaLAEOCARIDA. 



Neocarida. Palaeocarida. 



Integument solid and calcareous, or thin and chitinous. Integument usually chitinous. 



Usually in higher forms a cephalothorax, but in Phyllo- Head and abdomen alone; no thorax except in trilobites. 

 pods no genuine cephalothorax distinct from the abdomen. 



Eyes of normal form, rods and cones present, but no cor- Eyes with no rods and cones, but corneal lenses, 

 neal lenses. 



Two pairs of antennae. No antennae, either functional or morphological. 



Mandibles normal. No functional mandibles = gnathopods. 



Maxillae normal. No functional maxillae = gnathopods. 



Maxillipeds normal. No functional maxillipeds z= gnathopods. 



Gills on thoracic feet, or thoracic or abdominal feet them- Gills on the abdominal feet, 

 selves broad and thin, and serving as gills. 



Abdominal feet biramous. Abdominal feet biramous. 



Heart polygonal or tubular. Heart tubular, as in many Neocarida except Decapoda. 



Digestive canal with its three subdivisions of fore-, mid- Digestive canal homologous with that of most higher 



and hind-gut. Crustacea. 



Nervous system with a brain sending nerves to the anten- Nervous system with brain supplying eyes alone — first pair 



nae and eyes. of gnathopods supplied from oesophageal collar, in larva 



from suboesophageal ganglion. 



Oviduct opening at base of middle thoracic feet ; male out- Oviduct and male outlet situated at base of first abdominal 



let at base of 5th thoracic feet. feet. 



Metamorphosis often complete. Metamorphosis absent, or partial. 



Nauplius in some forms. No Nauplius. 



Zoea in Decapods. No Zoea. 



The difficulties which stand in the way of associating the Merostomata (thromng 

 out the Trilobites for the sake of clearness of statement) with the Crustacea, are : (1) 

 the nature of the limbs, and the absence of the pairs of antennae ; but it may be observed 

 that in the undifferentiated gnathopods of Limulus we have a parallel in the larval 



