396 ME. H. M. BERNAED ON THE 



ADnclidan type of muscles, that of the Arachnids is a specialization of the muscles of primitive 

 undifferentiated segments, starting fi'om a time when the intersegmental membranes were not drawn in 

 to form an endoskeletal system. 



17. The Central Nervous System. — The original form possessed a central nervous 

 system somewhat less specialized than that of the Spiders. The concentration of 

 locomotion at the anterior end of the body, and its liigh development, taken in connection 

 with the degeneration of the posterior region into a vegetative sac, would lead to a 

 crowding together of the ganglia of the locomotory segments along the floor of the 

 cephalothorax, while those of the abdomen degenerated, except where retained for 

 special functions (ef. the formation of tails, &c.). The translocation of the first pair of 

 appendages would easily lead to the wandering of their ganglia up the ocsojihageal 

 commissures until they assumed a supra-oesophageal position. From this primitive 

 difi'erentiation we can deduce all the central nervous systems of the Arachnida ; the 

 variations they present being always in harmony with the different degrees of longitu- 

 dinal compression of their cephalothoracic segments. 



No two central nervous systems could well be more unlike than those of Limulus and Scorpio. They 

 are extremes of opposite specializations. The brain has in Limulus retained the ventral position due to 

 the bending round ventrally of the prostomium, while in the Arachnids it has retained the primitive 

 dorsal position, and in Scorpio has been forced further backward dorsally than in any other Arachnid, 

 Ijy the crowding forward and upward of the ganglia of the first two pairs of appendages. Limulus 

 and Scorpio are thus wider apart in this respect than any other two Arthropods, each representing an 

 extreme position of the brain proper. I use the word " brain " to indicate those ganglia which, in a typical 

 Annelid, are supra-oesophageal, the optic and prostomial (and ? certain ganglia innervating the mid-gut) . 



18. The Eyes. — These, grouped originally on an ocular tubercle {cf. p. 392), perhaps 

 consisted of two large median and a certain number of accessory eyes, which latter have 

 very generally (except in the Aviculariidae) wandered away laterally on to the cephalic 

 lobes. 



The histological character of the primitive eyes is probably no longer ascertainable. From this point 

 of view, no value whatever can be placed upon a comparison of the eyes of Scorpio and those of Limulus. 

 We do not know whether the median eyes really correspond in the two, nor whether, in view of the 

 great differences which prevail among the eyes of Arachnids, those of Scorpio are primitive. Further, 

 any resemblance between the eyes would not necessarily mean any relationship between Limulus and 

 the Arachnids as Arthropods, since the primitive Annelidan ancestor possessed eyes, and some common 

 form of eye might have been transmitted to the two groups of descendants, the Arachnida and the 

 Palseostraca. 



19. The Oesophagus and the Sucking- Apparatus. — The position of the sucking-stomach 

 in the primitive form was in front of the nerve-mass (Galeodes, Scorpio, Thelyphonus, 

 Chernetids). This is in keeping with our description of the original distortion of the 

 anterior segments. The backward dorsal distortion of the 1st segment, while its sternal 

 surface was protruded to form the labium of the beak, naturally drew the oesophagus 



