PHYSIOLOGICAL SBRIBS. 



from iU postero-lateral surface, and upon its anterior face 

 i number of finger-like processes of doubtful function 

 (poasibly sensory). It ha* been shown that a special reflex 

 connection exists between the cerebral ganglion (a purely 

 ensory centre ?) and the motor centre that controls the 

 retractors of the introvert. Two long (11 mm.) circum- 

 OMOphageal connectives (from which the nerves for the 

 retractors of the introvert are given off) unite the cerebral 

 ganglion to tin- ventral cord. The latter for the first part 

 of iU course lies loose in the body-cavity accompanied by 

 a strand of muscle (mostly removed in the specimen) ; 

 altout 10 mm. in front of the nephridia it becomes closely 

 applied to the body-wall, and runs in this position between 

 two bundles of the longitudinal muscle-layer to the posterior 

 extremity of the body, where it terminates in a spindle- 

 shaped enlargement. 



So long as the ventral cord lies close to the body- wall it 



* off, about the middle of each circular muscle-band, a 

 pair of lateral nerves, that run between the circular and 



_onal muscle-layers towards the dorsal mid-line, but do 

 not meet to form a complete ring round tho body. In front 

 of tin- ncphridia the nerves come off more irregularly and 

 areenvelop.-d in >tnmd- of muscle; they supply the anterior 

 ond of the trunk and the introvert. No nerves arise 

 from the con! within the introvert. The cord is a mixed 

 motor and sensory centre in which impulses are slowly 

 propagated (100-200 nun. per second) in either direction. 

 The oerebral ganglion consists of a central mass of neuw- 

 pilr surrounded by unipolar ginglion-oelLi varying in size 

 from l-. r '> /i. and arrangi-d in fairly definite groups; a few 



tar cells occur in th< neighbourhood of the digitiform 



I'be -iib-tance of the ganglion i> permeated by 



a network of neuroglia fibres, and it is enclosed in a pro- 



f large ^teJlate cells (neuroglia cells?). The 



6Qfd fe composed of a fibrous core covered on it> ventral 



"urfaoe by ganglion-cell- ; it -bows no -ign of ganglionation 



OT lateral duplicity. It is ^mounded by a double sheath 



of nrurilemma, the space between the two being filled with 



' tho* around the cerebral ganglion. 



The posterior enlargement does not differ in structure from 



