NERVOUS SYSTEM. 403 



(1028.) But this change in the condition of the nervous system is 

 not only observable as we proceed from species to species, as they rise 

 higher in the scale of development ; similar phenomena are met with in 

 watching the progress of any individual belonging to the more perfect 

 families, as it advances from the embryo to its mature condition. Thus, 

 in the Cray-fish (Astacus fluviatilis), Eathke* observed that, when first 

 perceptible, the nervous system consisted of eleven pairs of ganglia, 

 perfectly distinct from each other, and situated on each side of the 

 mesial line of the body. The first six pairs then unite transversely, so 

 as to form as many single masses, from which the nerves of the man- 

 dibles and foot-jaws emanate ; while the five posterior, from which the 

 nerves of the ambulatory extremities are given off, remain separate. 

 Such is the state at birth, or on leaving the egg ; but further changes 

 occur before the Cray-fish arrives at maturity. The four anterior 

 ganglia, which supply nerves to the mandibles and foot-jaws, are, by 

 degrees, all consolidated into one mass, and the fifth and sixth likewise 

 coalesce, while the other pairs continue permanently distinct. The 

 reader will at once recognize the resemblance between these changes 

 and those already described as taking place during the progress of 

 evolution in the Caterpillar : the same great law is, in fact, in operation 

 in both cases, and the same results are obtained from the completion of 

 the process f. 



(1029.) From a review of the above facts, Milne-Edwards and 

 Audouin arrived at the following conclusions : 1st. That the nervous 

 system of Crustacea consists uniformly of medullary nuclei (ganglions), 

 the normal number of which is the same as that ^of the segments or 

 rings of the body. 2nd. That all the modifications encountered, whether 

 at different periods of the development or in different species of the 

 series, depend especially on the more or less complete approximation of 

 these nuclei, and on an arrest of development in some of their number. 

 3rd. That approximation takes place from the sides towards the mesian 

 line, as well as in a longitudinal direction. 



(1030.) In the Crab, the distribution of the nerves is briefly as 

 follows : The encephalic mass, or brain, which still occupies its position 

 above the oesophagus, and joins the abdominal centre by two long cords 

 of connexion (fig. 206), gives off nerves to the eyes, and muscles con- 

 nected with them, as well as to the antennae and neighbouring parts. 



(1031.) Near the centre of each division of the nervous collar that 

 surrounds the oesophagus is a ganglionic enlargement, from which arises 

 a nerve that runs to the mandibles, and also a very important branch, 

 apparently the representative of the neruus vagus of insects. This, after 



* " Untersuchungen iiber die Bildung des Flusskrebses," Ann. des Sci. Nat. t. xx. 



t For a minute account of the arrangement of the nervous system in these ani- 

 mals, the reader is referred to the Cyclopaedia of Anatomy and Physiology, art. 

 " CRUSTACEA," by Dr. Milne -Edwards. 



