NERVOUS SYSTEM. 401 



all the interspaces left between the various viscera, as well as the still 

 smaller lacunae situated among the muscular fibres or underneath the 

 skin : but the heart, instead of opening immediately into this system of 

 intercommunicating cavities, as among the true insects, is continuous 

 with a special system of tubes, the walls whereof are well defined, and 

 of which the peripheral branches ramify in the substance of all the 

 organs of the body, thus constituting a very complete arterial system, 

 although, by their ultimate ramifications, the centrifugal vessels thus 

 formed become continuous with and lost amongst the interstitial lacunae 

 of the body, which in their turn communicate with more considerable 

 cavities situated between the viscera ; so that the blood ejected by the 

 heart and arteries, arriving in the last ramifications of those tubes, 

 escapes into the general interstitial lacunary system, by the intermedium 

 of which it returns towards the heart. Thus the circulating fluid is 

 brought into direct contact with all the viscera, and fills up the abdominal 

 cavity, so that not until after it has passed through the respiratory 

 apparatus does it again find itself enclosed in vessels properly so called. 



(1024.) As might be anticipated from an examination of the external 

 configuration of the different families comprised in the extensive class 

 we are now considering, the nervous system is found to pass through 

 all those gradations of development which we have found gradually to 

 present themselves as we have traced the Homogangliata from the lowest 

 to the most highly organized types of structure. In the most imperfect 

 Crustacea, indeed, we find a simplicity of arrangement greater than any 

 hitherto pointed out even in the humblest Annelida a disposition of 

 parts which, theoretically, might have been expected to exist, but has 

 only been distinctly recognized in the class before us. 



(1025.) "We have all along spoken of the nervous centres of the 

 Articulata as arranged in symmetrical pairs, although in no example 

 which has yet occurred to our notice have we been able strictly to point 

 out the accuracy of such a view of the subject. The two lateral masses 

 of the supra-o3sophageal ganglion are found united into one brain in 

 the humblest forms of annulose animals ; and even in the ganglia form- 

 ing the ventral series, although we might presume each to be composed 

 of two symmetrical halves, the divisions are most frequently so inti- 

 mately blended, that their distinctness is not susceptible of anatomical 

 demonstration. In some of the Crustacea, however, among those species 

 which have the segments of their external skeleton most perfectly 

 separate and distinct, the nervous system is found to present itself in 

 such a condition that the division into lateral halves is perfectly evident; 

 and from this condition their progressive coalescence may be traced, 

 step by step, until we arrive at a state of concentration as remarkable as 

 that already noticed in the most elevated of the Arachnidans. It is to 

 Milne-Edwards and Audouin that we are indebted for the interesting 

 particulars connected with this part of our subject ; and the results of 



