38 A. S. PACKARD, JR., ON THE ANATOMY • 



make out the general form of the main nervous cord. Plate 3, fig. 3, shows the general 

 relations of the cord to the body. It is large and broad, with three well-marked pairs 

 of consolidated ganglia in the abdomen, the two basal ones supplying the nerves for the 

 first and second abdominal feet. There are in the cephalothorax six pairs of consolidated 

 ganglia, the commissures being as yet undeveloped ; the ganglia are indicated by the 

 minute openings in front of and behind each pair of ganglia. The ganglia of the first pair 

 of feet could be clearly distinguished ; the brain or cephalic ganglion is probably repre- 

 sented at fig. 3 I ; fig. 3 a, the same enlarged. The number of ganglia, throwing out the 

 brain, is nine, corresponding to the six pairs of cephalothoracic feet and tlie two abdominal 

 segments, there being at this stage but two pairs of appendages in the abdomen. 



The next important stage of development is seen in longitudinal sections of the larva 

 after hatching, and when the digestive canal is marked out. To show the ganglia best, 

 the section should be made on one side of the median line of the body, so as to pass 

 through the middle of the ganglia on one side. Plate 3, fig. 2, shows a section thus made 

 and stained with carmine ; the nervous ganglia remaining white are very clearly indicated ; 

 the commissures are not shown, but they are now developed, since the ganglia are mostly 

 separate. 



Now if we make a longitudinal section of the young horse-shoe crab when a little over 

 an inch long, the disposition of the nervous cord is exactly as in the full grown individual, 

 as figured by A. Milne-Edwards ; see also our representation on plate 3, fig. 1, hr, oe r. 

 The nervous ganglia are then united into a nearly continuous nervous collar, the opening 

 in front being filled up by the brain or cephalic ganglion.^ 



Turning now to the nervous system of the larva (plate 3, fig. 2), the section here figured 

 shows a most important and interesting difference as regards the ganglia which supply 

 nerves to the appendages of the cephalothorax. They are at this time entirely separate, 

 the spaces between the four posterior ones, which are connected by commissures, being as 

 wide as the ganglia themselves are thick. There are behind the oesophagus six ganglia, 

 corresponding to each of the six pairs of gnathopods ; while the brain is rather larger 

 than the others, and the first post-oesophageal ganglia are the smallest of the six, corres- 

 ponding to the more diminutive size of the first pair of gnathopods. 



Reference may also be made here to plate 5, fig. 8, which shows the mode of origin of 

 the nerves distributed from the first post-oesophageal ganglion to the feet; this section 

 certainly very clearly demonstrates that the first pair of gnathopods belong with the post- 

 oral series, that they can in no sense be regarded as homologues of the antennae of 

 other Arthropods, and that in fact there are no antennae in Limulus, and without doubt 

 in the Merostomata in general. But this subject has been ali-eady discussed in the 

 chapter on morphology. 



It is not until after the second moult that the adult condition of the nervous system is 

 attained, as Dr. Dohrn^ has figured the separate ganglia in a larva which had evidently 

 moulted once, the abdominal spine being well developed. This is certainly an interesting 



1 For the nature of the brain and the oesophageal gang- ^ Dohrn. Zur Embryologie und Morphologie des Limulus 



liouic collar, the reader is referred to the section of this polyphemus. He also represents the fourth pair of abdominal 

 paper on the structure of the adult brain. appendages ; the larva has but three before the first moult. 



