230 



NATURAL HISTORY. 



essential constitution as all the others forming the body. In the Annelids or Ringed Worms this 

 theoretical condition is very nearly realised, that is to say, with the exception of a few modified 

 segments, all the somites of the body ai-e exactly equivalent, at least so far as regards the particular 

 systems of organs which are related to the segmentation. These must necessarily be the organs 

 produced from the outer germinal layer during the development of the embryo in the egg, namely, the 

 integument, in which division into segments is so strongly marked, the central nervous system, and 

 any organs directly connected with the skin. In perfect insects we find the division into segments 

 sufficiently clear, but when we come to the interior the case is frequently very different. But in 

 the larvae of insects the conditions presented by the worms are almost exactly reproduced ; and as, 



notwithstanding the change of form, there is no denying 

 the individual identity of the perfect Butterfly and its 

 Caterpillar, for example, we must accept the peculiarities 

 presented by the former as -produced solely by the modifi- 

 cation of parts present in the latter. This applies especially 

 to the central nervous system, which often differs very 

 greatly in the same insect at different stages of its 

 existence. 



Thus in most larvse and in many perfect insects we 

 find the central nervous system forming a more or less 

 regular and uniform chain, extending from one end of 

 the body to the other. In the head there are four nervous, 

 knots, or ganglia as they are called, two of them (which, 

 however, are often united, although even then generally 

 recognisable) situated above the oesophagus, and forming 

 together what is frequently called the brain or the cerebral 

 ganglion; and two, which are almost always amalgamated 

 together, placed beneath the oesophagus. The upper and 

 lower ganglia are united by short nervous cords (commis- 

 sures) on each side, so that the oesophagus is surrounded by 

 a sort of nervous collar or ring, more or less enlarged above 

 and below. From the lower enlargement (that beneath 

 the oesophagus) a pair of slender nervous cords, frequently 

 united to form a single one, run backward into the thorax, 

 and are continued throughout the length of the body, 

 bearing in each segment, until they nearly reach the end of 

 the abdomen, an enlargement or ganglion, which often shows 

 traces of being composed of two halves (Fig. 9). In this 

 way we get a chain of ganglia united by slender com- 

 missures, which may amount to eleven in number, exclusive 

 of those in the head, that is to say, one in each segment 

 of the thorax, and eight belonging to the abdomen. The 

 maximum number of body-ganglia thus conies within 

 one of that of the fully developed body-segments. But from this uniform development of the 

 central nervous system, the departures are exceedingly numerous and varied, in relation chiefly 

 to the suppression or amalgamation of the segments in the perfect insects, the general tendency 

 being to shorten the nervous chain, and thus confine the central masses more and more to the 

 anterior part of the body, in accordance with the general direction in which specialisation takes 

 place in this, and indeed in other classes of animals. Thus fusion of the ganglia of the meso- 

 thorax and metathorax, or of all the three thoracic ganglia into a single mass, may occur, while the 

 abdominal ganglia still remain separate (Fig. 10) ; then the latter may unite into a mass which joins 

 the thoracic mass, or the mass formed by the two hinder thoracic ganglia, to constitute a long single 

 central nervous organ in which the original constitution out of separate ganglia is almost 

 wholly obliterated. The nerves which in insects with a regular chain of ganglia issue from 



Fig. 9. NERVOUS SYSTEM OF LAKVA OF BEE. 



