38 SUMMABY OF CUBRENT BESEABCHES BELATING TO 



into the ordinary " nerve-nuclei." At the origin of the second pair of 

 sensory nerves, there begins, above the central canal, a group of medium- 

 sized multipolar ganglion-cells, which extends to the region of the fifth 

 sensory nerve. In front of the posterior end of this dorsal group there 

 begins on the ventral side a similar layer of medium-sized, but on an 

 average rather smaller, ganglion-cells, which appear to be unipolar or 

 bipolar. The two groups are connected by lateral ganglion-cells. 

 These groups must be included in the brain. The beginning of the cord 

 is marJced with tolerable exactness by the position of the most anterior giant 

 ganglion-cell, and the first five pairs of sensory nerves are to be regarded as 

 cerebral. Previous investigations are then noted. 



The sensory nerves, which alternate with one another, consist, like the 

 dorsal portion of the central system, of delicate fibres. Especially near 

 their origin " nerve-nuclei " are imbedded in the nerves. The spinal 

 ganglia of higher Vertebrates are here represented by aggregations of 

 these nerve-nuclei. They are more abundant on the posterior nerves. 

 The 64th or most posterior pair of sensory nerves, behind the last 

 muscle-segment, appears to have been overlooked by previous investi- 

 gators. 



The motor nerves, alternating with the sensory, and arising from the 

 ventral side, consist of fibres somewhat less thick than the medium-sized 

 elements which accompany the giant fibres of the ventral region. Two 

 or more fibres are often apposed. Forking and lateral branching 

 occur. The internal connection with the nerve-elements of the central 

 system is still uncertain. Peripherally the motor fibres enter individu- 

 ally into connection with the muscle-plates. Details for the different 

 regions are noticed. The motor-fibres often exhibit marked transverse 

 striation like that of the longitudinal musculature. Most, however, are 

 homogeneous. It appears most probable that the motor nerves are really 

 the apparatus for the motor stimulus of the longitudinal musculature. 

 It is possible that the transverse musculature may be innervated by 

 " sensory " nerves. 



B. INVERTEBRATA. 



Mollusca. 

 y. Gastropoda. 



Eyes of Gastropods and of Pecten.* — Dr. G. Kalide has a preliminary 

 report on his investigations into the minute structure of the eyes of 

 Gastropods and of Pecten. In the Prosobranchiata, of which Nassa may 

 be taken as the type, the optic vesicle is separated from the surrounding 

 connective tissue by a transparent basal membrane, into which the 

 neurilemma of the optic nerve passes. The fibres of the optic nerve 

 spread out in all directions ; internally to them is the cellular layer of 

 the retina, the components of which are arranged radially to the centre 

 of the eye. The pigment layer is external to the zone of rods. The 

 central cavity of the vesicle is filled by a transparent mass, which forms 

 a lens anteriorly and a gelatinous vitreous body posteriorly. 



The innervation of the retina was first made out distinctly in Ptero-' 

 trachea coronata, where the retinal cells gradually diminish at their outer 

 ends and pass gradually into nerve -fibres, which are lost in the expansion 

 of the optic nerve; this arrangement has already been detected by 



* Zool. Auzeig., xi. (1S88) pp. 679-83, 698-703. 



