84 ANATOMY OF THE ENOPLA. 



immediately behind the ganglia, which, if not an original condition, may be due either 

 to comparative immunity from pressure, or a tougher investment. In transverse section the 

 nerves present a delicately granular appearance, from the ends of the fibres. No one who has 

 seized on such specimens as N. gracilis in semi-contraction (though unwrinkled), and drawn 

 them out to treble the length and upwards, can doubt the peculiar elasticity pertaining to the 

 lateral nerves in these animals. According to Duges, indeed, Darwin thought nervous substance 

 was contractile. 



The nerve-trunks are placed by M. de Quatrefages " between the external longitudinal and 

 internal transverse muscular fibres " of the body-wall ; a position which may in some respects 

 apply to the Anopla, but is inapplicable to the present group. Prey and Leuckart mention that 

 the lateral nerves lie inside the muscular coats; but while indicating the different arrange- 

 ments of the " brain" in Tetrastemma and Borlasia, they do not explain the distinction in regard 

 to the position of the nerve-trunks in these genera. 



9. Eye- Specks. 



The eye-specks are situated beneath the superficial muscular coat ; thus, in many spirit- 

 preparations none are visible until the dermal layers are removed. In most examples they 

 consist of simple masses of black pigment. So far as has been seen, only four British Enopla, 

 viz., A.pulcher, A. spectahilis, the Zetlandic variety of Tetrastemma Candida, and Prosorhochmus 

 Claparedii, show a special arrangement in their eye-specks, and even in those it is of a very 

 elementary character. In the former the pigment in each eye-speck is grouped into a globular 

 mass with a smooth outline, and in the living animal under examination there is frequently a clear 

 patch in the centre (Plate XIV, fig. 15); but, though the mass retains its well-defined outline 

 after the removal of the pigment by caustic potash, no capsule could be made out as a separate 

 structure. The brownish Zetlandic variety of Tetrastemma Candida, again, has the anterior pair 

 of eyes considerably larger than the posterior, the former, moreover, often presenting a crescentic 

 margin in front. The pigment-grains are arranged in the same manner in both, but the outline 

 of the posterior pair is more irregular and less finished than that of the anterior. Each of the 

 latter has a somewhat pale space in front of the crescentic margin, so that the eye appears 

 to be furnished with a capsule or lens, but I have not been able to demonstrate either, 

 and conclude that it is a pallor due to the greater translucency of the tissues in front 

 of or over the pigment. Such, doubtless, may indicate a step in the formation of a lens. A 

 clear globule has been seen in connection with the anterior eyes of Prosorhochmus Claparedii, but 

 it may have been a simple cutaneous structure. 



In N. gracilis and others, a few of the eye-specks are frequently connected together by 

 bridges of the pigmentary substance. Though a pale portion is sometimes seen in the specks 

 of the former, there is no evident capsule or lens (Plate XIV, fig. 2). The large black pigment- 

 patch on the snout of Tetrastemma melanocephala often includes the first pair of eyes ; while in 

 T. vermicula the eyes of each side are connected by a longitudinal band of dark pigment, which 

 frequently becomes crescentic in contraction. Mediterranean Tetrastemmce with " lenses" to the 

 eye-specks have been noticed by Delle Chiaje, Kolliker, GraefTe, and A. P. Marion. 1 I have 

 found the specks disappear from adult specimens of Prosorhochmus Claparedii, after a year's 

 confinement. 



1 ' Ann. Nat. Hist./ 4th ser., vol. iv, p. 136. 



