PREVIOUS INTERPRETATIONS OF CNIDARIA 127 



developed in one layer, or in the ectoderm, only, as we are 

 now used to saying (thus subepithelially or under the skin), 

 but it also occurs deeper, in the intermediate layer (C.A. Pantin, 

 1952). Furthermore, the nerve processes do not respect in 

 this connection the limit between the two layers (the support- 

 ing lamellae or interstitial lamellae). This fact, taken separately, 

 does not seem to be very surprising, yet it becomes clear if 

 we make a comparison of the nervous system of Cnidaria with 

 the one that can be found in Turbellaria. 



Yet before we begin to deal with this problem, let us first 

 make a cursory survey of the nervous systems that exist in othei 

 animal types that have also adopted the sedentary way of life. 

 It can be observed in such sedentary animal types that have 

 evolved from freely living ancestors that already possessed a 

 wxU developed central nervous system, that the main centre, the 

 brain ganghon, has been preserved with obstinate tenacity, 

 long after their transition to the sessile way of life, and it can 

 still be found when other parts, especially the ventral nerve 

 cord, had disappeared, and when the peripheral nervous system 

 only has been kept. A centre has been preserved even in those 

 cases where sensory organs have not been developed in great 

 abundance. As a rule we find the supraoesophageal ganglion 

 preserved. This centre has disappeared, as it seems, in Endo- 

 procta only and this w- as probably due to the part of the body 

 which was used by these animals when they began to attach 

 themselves to the bottom. On the other hand, we find the 

 suboesophageal ganglion preserved and it functions as a brain 

 ganglion. In addition to these, the brain ganglion with its 

 ventral extension (the nerve cord) has been lost in Echino- 

 dermata when they definitely adopted the sedentary way of 

 life ; in these a peripheral nerve net only has been preserved. 

 We can observe, as a fine analogy to the medusae of Cnidaria, 

 that no younger types of Echinodermata have developed a 

 nerve centre similar to a brain ganglion, not even in those 

 cases where they have evolved secondary ambulatory abilities. 

 We can find in the freely moving Echinodermata, as well as in 



