INVEKTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 639 



The arrangement tlius brought about is not unlike that which obtains 

 in P. Grajfii, where all the nerves given off from the brain are con- 

 nected together by a circular commissure ; but in Tristomum there is 

 not any direct continuation between the commissures in the posterior 

 region. The foui'th pair of nerves passes furthest backwards ; and 

 these may be seen to consist essentially of a feebly developed dorsal, 

 and of two well-developed ventral nerves on either side. Of the ventral 

 trunks, one is peripheral and external, the other internal ; these again 

 unite in the region of the great ventral sucker. 



After describing in detail the distribution of these nerves, the 

 author passes to the consideration of the minute structure of the nervous 

 system ; the examination of which is greatly aided by the large size 

 of the ganglion-cells and of the nerve-elements, as well as by the dis- 

 tinctness of the nerve-tracts. A good transverse section of a ventral 

 longitudinal nerve exhibits par excellence the spongy character of 

 these fibres ; of these there is a network, and in the midst of them 

 there are a number of lumina of various sizes ; these all contain some 

 coagulated protoplasm, and in the larger ones nucleated cells are also 

 to be distinguished. Careful examination of a number of sections 

 reveals the existence of a number of tubes {neurilemma), and of nerve- 

 fibres enclosed in these tubes. It would seem that, during life, the 

 fibre completely fills the tube. It is concluded that, histologically at 

 any rate, the cerebrum is nothing but a specially and highly developed 

 transverse commissure, which indicates its relation to the central 

 nervous system by being composed largely of ganglion-cells. 



The eyes, which are extremely simple, consist of (1) an aggrega- 

 tion of pigment covering in (2) a spherical or oval refracting body, 

 which in the anterior eyes is directed backwards, and in the posterior 

 forwards. Connected with this there is (3) a typical ganglionic cell 

 which forms the retina. (4) A spherical bundle of the dorso-ventral 

 muscles appears to act as muscles for the eye. 



Small peripheral nerve-centres appear to be represented by large 

 cells which, scattered through the body, are best developed in the 

 neighbourhood of those regions in which the musculature is best 

 developed. 



Pleurocotijle scombri. — The nervous system of this creature is the 

 next subject of Dr. Lang's investigations. No eyes are hero develojied ; 

 the cerebral mass is delicate, and is made up of finer fibres ; the most 

 distinct nerves belong to four series : — 



(1) A pair which pass forwards to the suckers. 



(2) A i)air which i)ass upwards — dorsal nerves — but which could 

 not be traced for any great distance. 



(3) A i)air, which pass outwards and upwards, and are soon lost ; 

 and (4) A pair, better develoijed, of longitudinal trunks, which take a 

 backward course along the ventral siii-fuce. 



JJislomida. — The examples of this group which were examined 

 were Uistoinum nijrojlavum, and 1). hcjxilicum. The ceutiuil nervous 

 Bysteni of the former has the tyi'ieiil position, between the oral sucker 

 and the pharynx. From its ujtptr portion tliero is given oU'dorsally 

 on either side a nerve for the oral sucker, and a nerve which j>asecs 



