gjg Comparative Animal Physiology 



turns toward the light, but when illuminated from the operated side it turns 

 away from the light.'^-"^' -^'^ Apparently the scattered photoreceptors set up 

 a homolateral reflex in the ventral ganglia, a positive response to light; the 

 tracts from the prostomium and first two segments, however, cross in the 

 brain and cause a negative response. The brain normally dominates, but at 

 very low light intensities a positive response may occur. 



The brains of some polychaete worms are complex and have lobes corre- 

 sponding to the specialized forebrain centers of arthropods. ^^^ These lobes 

 (corpora pedunculata) are best developed in free-swimming polychaetes 

 which have cuticular eyes (Fig. 306, A); they do not occur among oligo- 

 chaetes. An investigation of integrative functions of the polychaete brain 

 should be very interesting. 



Arthropods. There is much variation in complexity of the brain among 

 different groups of arthropods. In general there are three regions: the proto- 

 cerebrum, the deutocerebrum, and the tritocerebrum. The bulk of the lateral 

 portions of the protocerebrum is devoted to vision centers which are directly 

 connected to the eyes; the middle and anterior portions contain the associa- 

 tion areas, the protocerebral bridge, the central body, and the large cellular 

 corpora pedunculata (Fig. 306, B, C). There are no corpora pedunculata 

 among some lower Crustacea. The deutocerebrum usually lies ventro-an- 

 teriorly and may contain large antennal centers as well as "olfactory" lobes. 

 The tritocerebrum lies behind and customarily gives rise to certain nerves 

 going to mouthparts as well as to the stomatogastric nerves; the tritocerebrum 

 is continuous with the circumesophageal connectives. 



In view of the complex instinctive behavior and limited but real modifi- 

 ability of behavior in arthropods, especially among insects, an understanding 

 of the function of diflferent parts of the brain should be useful. Hanstrom^^-^ 

 has compared the relative portions of the brain mass occupied by different 

 regions in various groups as in Table 78. 



Since the days of Aristotle, the observation of behavior of arthropods from 

 which parts of the body have been removed has been a favorite pastime of 

 naturalists. Reviewers^'" • ^"- have listed fifteen papers dealing with brain 

 operations in crustaceans, sixteen on insects, and five on myriapods. The 

 findinos are roughly similar in all of these studies. 



Removal of any part of the brain which receives tracts from any sense 

 organ is equivalent to removing that sense organ. Immediately after removal 

 of a sense organ or injury to the brain, there may be a shock reaction of gen- 

 eral incoordination. Thereafter, specific behavioral deficiencies appear. Most 

 arthropods can eventually compensate for the removal of one eye or of an 

 antenna, whereas injurv to the brain leaves a more permanent behavior de- 

 ficiency. If both circumpharyngeal connectives are transected, reflexes of the 

 head, such as antennary movements, remain; otherwise the effect is like total 

 brain removal. Spontaneous directed locomotion ceases, although locomotion 

 is possible under stimulation. Locomotion is less impaired by brain removal 

 in some species (_Carcinus and Eupagurus^ than in others (AsfacMs). Mouth- 

 parts may move, but usually there is no coordinated feeding. Some animals 

 can chew but cannot swallow after brain removal. Righting and leg reflexes 

 arc unimpaired. Usually there is extensive random activity of legs. 



When one circumesophageal connective is cut, or one lateral half of the 



