268 REPORT — 1851. 



the opposite side; or in those of segments at a greater or less distance, 

 according to the point at which the efferent fibres leave the cord." The 

 degree of development exhibited by the nervous system in this class of ani- 

 mals, as in all others, bears a direct ratio to that of the muscular system, the 

 distinctness of the organs of sense, and the complication of the pedal and 

 tactile appendages. When these last are entirely wanting, the ventral cord 

 exhibits no ganglia ; when they are highly developed, these are completely 

 and powerfully formed. This rule however is liable to exception, since the 

 nervous systems of the Leech and the Earth-worm are more prominently gan- 

 glionic than that of the Nereids. 



In the Leech the nervous system consists of a long series of minute ganglia 

 joined by internuncial cords ; these ganglia amount to about 24* in number. 

 The anterior pair, or that immediately beneath the oesophagus, is larger than 

 the rest, forming a minute heart-shaped mass, which is circled by a delicate 

 nervous collar embracing the gullet, with two small nodules of neurine 

 situated upon the dorsal aspect of the mouth. The diminutive bodies last 

 mentioned constitute that portion of the nervous system most immediately 

 connected with sensation ; for while the nervous filaments given off from the 

 abdominal ganglia are distributed to the muscular integuments of the body, 

 the nerves which issue from the supracesophageal pair supply the oral sucker 

 where the organs of sense are situated. In all homogangliata, indeed, it is 

 exclusively from this cephalic pair of ganglia that the nerves appropriated to 

 the instruments of the senses are derived ; the name of brain may therefore 

 be aptly applied to this part, since it is the physiological correlate, if not the 

 anatomical homologon of the cerebral mass of more highly organized beings. 

 In consequence of the rudimentary character of these centres, the apparatus 

 of sensation associated therewith must be proportionably rudimentary. The 

 material machinery of an organ of sense can only be utilized by a corre- 

 sponding amount of nervous influence, and where the latter exists the former 

 is commonly present. In the Hirudinis, as in other Annelida, distinct ocelli 

 have indeed been described by several anatomists. Although characterized 

 by the utmost simplicity of structure, they are stated nevertheless to present, 

 in the degree of their development, a proportion corresponding to the condition 

 of the cerebral centres with which they are in relation. The eyes of the 

 Leech are eight or ten in number, and are easily detected by the assistance of 

 a lens, under the form of a semicircular row of blaclc points, situated above 

 the mouth upon the sucking surface of the oral disc, a position evidently 

 calculated to render them efficient agents in detecting the presence of food. 

 The structure of these simple eyes, according to Prof. Miiller, does not as 

 yet present any apparatus of transparent lenses adapted to collect or con- 

 centrate the rays of light ; but each ocellus or visual speck would seem to be 

 merely expansions of the terminal extremity of a nerve derived immediately 

 from the brain, spread out beneath a kind of cornea formed by a delicate 

 and transparent cuticle ; behind this is a layer of black pigment, to which 

 the dark colour of each ocular point is due. These ocelli are detectable in 

 nearly all Annelida in ultimate organization ; in all instances, however, they 

 fall under the description above presented. In the Nemertinidte these or- 

 ganules are prominently visible, amounting to 12 or \6 in number. The 

 Nereids are distinguished for the large size of the eyes, which stand in relief 

 at the bases of the tentacles as two black spots. Those who have watched 

 the habits of the Nereids will scarcely doubt that they are gifted with the 

 power of discriminating external objects, of making towards some point, and of 

 avoiding others. In the absence of such an optical arrangement as may be 

 fitted to collect the rays of light, the physiologist, however, can form no con- 



