Fundamental Types of Organizatiwi. 86 



bands of nervous substance. To the first division belong those 

 animals whose bodies consist of articulated rings, comprising the 

 Crustacea, insects, and worms ; to the second, the molluscous ani- 

 mals and zoophytes. 



The distinguishing neurological mark between the Crustacea 

 and insects, and the worms, is the inequality in the several por- 

 tions of the abdominal ganglia. This is obvious in the higher 

 insects at the first glance. Their thoracic ganglia always differ 

 in form and size from the abdominal. The difference is not so 

 marked in the genera nearest to the worms, — the Millipes, Sco- 

 lopendrum, and Julus, But even here the gangha, from which 

 arise the nerves supplying the organs of generation, are differ- 

 ent in size from the others ; whereas in the worms, no other 

 distinction is observable between the ganglia than a general de- 

 crease in size and relative distance towards the posterior end of 

 the animal. They have, in common with most Crustacea and 

 insects, the possession of but one series of ganglia situated in the 

 middle line of th& abdomen, and composed of a symmetrical 

 rifjht and left half. A deviation from this structure exists in 

 the phalangiae which possess ganglia on both sides of the abdo- 

 men, but united with one another only by simple filaments. 



This first division passes into the second by the cirrhipedes, 

 which, in the articulated posterior part of their bodies, possess a 

 series of ganglia, as in the former animals, but in the upper por- 

 tion of their cerebral ring, no hemispheres immediately connected 

 together. Among the mollusca, there are families distinguished 

 from all other animals by the want of symmetry of their brain. 

 In those where the cerebral ring is symmetric, it has no central 

 mass. Nothing certain is known of the nervous system of the 

 zoophytes. But from the general radiated structure of this class 

 of animals, it is probable that it is merely a simple cord, sur- 

 rounding the orifice of the alimentary canal, or an aggregate of 

 ganglia united in the form of a ring, and which give off nerves 

 in a radiated manner. 



Much is still wanting to carry out this neurological classifica- 

 tion in all its details ; and even did we possess an adequate mass 

 of observations, a more uniform division could hardly be pro- 

 posed. It will, however, be by no means out of place to trace 

 the concordance of the characters deduced from the nervous 



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