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



The thoracic ganglionic mass of the Arachnida is likewise not homologous with the 

 central cephalothoracic nervous system of Limulus. The thoracic mass in the former 

 type sends off nerves to the maxillae, or second pair of mouth-appendages, and also to 

 the four pair of limbs, and from this mass the abdomen, including the spinnerets (in our 

 view morphologically limbs), is supplied with nerves; there being no ganglia in the 

 abdomen of any spiders (Araneina) as yet known. On the contrary, the oesophageal 

 collar of Limulus supplies the nerves for the six cephalothoraic appendages alone (and 

 this seems strong proof that these gnathopods should be regarded as either mouth-parts 

 alone, or partly mouth appendages, and partly thoracic appendages), while there is a 

 chain of six ganglia in the abdomen. Here, however, it should be borne in mind that in 

 the scorpions there is a chain of abdominal ganglia, so that in this respect there is an 

 interesting analogy between Limulus and the Pedipalpi. So far, however, as concerns 

 the brain and thoracic mass, there seems to be a lack of homology in the two types of 

 nervous system of Limulus and Scorpio. 



In the mode of early development, Limulus resembles the Arachnida, but also in the 

 embryonal membranes the insects, while it also recalls the development of certain Crus- 

 tacea, notably Apus, as we attempted to show in our first memoir. 



The fourth point of comparison, i. e., between the gUls of Limulus and the pulmonary 

 branchiae of spiders seems far-fetched. The gills and mode of respiration of Limulus are 

 thoroughly crustacean, the gills being certainly hot homologues of the "lungs" of the 

 air-breathing spiders, which are tracheal sacs, formed by modified tracheae, and opening 

 externally by stigmata. 



From any point of view, developmental, anatomical or physiological, the relations 

 of Limulus and its fossil allies to the Arachnida seem purely those of analogy, the fund- 

 amental differences being such as characterize and separate the Tracheate from the Bran- 

 chiate Arthropods ; the differences are so fundamental as to suggest the idea that the two 

 types probably had a different origin, i. e. from some vermian ancestors. 



In order to epitomize the differences and resemblances between the Merostomata and 

 Arachnida, we have prepared the following tabular view : 



Comparison of the Merostomata with the Arachnida. 

 Arachnida. Merostomata {Limulus). 



Head in adult soldered to thorax. Head separate from hind body. 



No compound eyes. Compound eyes. 



No antennae or morphological equivalents. No antennae or morphological equivalents. 



Mandibles on hexapodous type. Only their morphological equivalents (gnathopods). 



Maxillae with a palpus, on hexapodous type. " " " 



Four pairs of thoracic legs on hexapodous type. No true thoracic legs; the gnathopods representing the 



mouth-parts and possibly the thoracic legs. 

 No functional abdominal legs, the spinnerets being, how- Six pairs of swimming respiratory legs, on the Crustacean 



ever, modified legs. type. 



Digestive canal on hexapodous type with a voluminous Digestive canal on Crustacean type, with a voluminous 



liver, and urinary tubes. liver, but no urinary tubes. 



Brain formed of two pairs of ganglia supplying eyes and Brain formed of a, single pair of ganglia, supplying eyes 



mandibles. alone, and free from the suboesophageal ganglion in embryo 



and adult. 

 Maxillae and thoracic legs supplied from a concentrated Gnathopods supplied from a concentrated ganglionic oeso- 



postoesophageal ganglionic mass. phageal ring. 



No abdominal ganglia in spiders, but present in scorpions. Six abdominal ganglia, much as in Crustacea. 



