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 chxss 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 tlie crustacean nature of Limidus 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 Euryjjterida being, according to H. Woodward, attached side by side, like the 

 teeth of a rake ; while the mode of respiration, as seen on 

 crustacean; (2) the resemblance of the cephalo thorax of Limulus 

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

 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 Trilobita) are shown in the following tabular view : — 



Comparison op Normal Crustacea (Neocarida) with Limulus and other Palaeocarida. 



plate 1, is truly 

 to that of Ai)us ; 

 of the Nauplius 



Neocarida. 



Integument solid ami calcareous, or thin and chitinous. 



Usually in higher forms a cephalotborax, but in Phyllo- 

 pods no genuine cephalothorax distinct from the abdomen. 



Eves of normal form, rods and cones present, but no cor- 

 neal lenses. 



Two pairs of antennae. 



Mandibles normal. 



Maxillae normal. ' 



Maxillipeds normal. 



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

 S'jlves broad and thin, and serving as gills. 



Abdominal feet biramous. 



Heart polygonal or tubular. 



Digestive canal with its three subdivisions of fore-, mid- 

 and bind-gut. 



Nervous system with a brain sending nerves to the anten- 

 nae and eyes. 



Oviduct opening at base of middle thoracic feet ; male out- 

 let at base of 5th thoracic feet. 

 Metamor])hosis often complete. 

 Nauplius in some forms. 

 Zoea in Decapods. 



Palaeocarida. 

 Integument usually chitinous. 

 Head and abdomen alone; no thorax except in trilobites. 



Eyes with no rods and cones, but corneal lenses. 



No antennae, either functional or morphological. 

 No functional mandibles = gnathopods. 

 No functional maxillae ^ gnathopods. 

 No functional maxillipeds r= gnathopods. 

 Gills on the abdununal feet. 



Abdominal feet biramous. 



Heart tubular, as in many Neocarida except Decapoda. 



Digestive canal homologous with that of most higher 

 Crustacea. 



Nervous system with brain supplying eyes alone — first pair 

 of gnathopods supplied from oesophageal collar, in larva 

 from suboesophageal ganglion. 



Oviduct and male outlet situated at base of first abdominal 

 feet. 



^Metamorphosis absent, or partial. 



No Nauplius. 



No Zoea. 



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

 out the Triloljites 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 m the larval 



