APPENDIX. 285 



in number according to the species, and which are considered by 

 Miiller and others as organs of vision. We are not able, with our 

 small microscope, to discover any peculiar organization in them, yet 

 there is reason (notwithstanding the objection of Lamarck, founded 

 on the want of optic nerves and a nervous system *) to assent to this 

 opinion, from the exact resemblance of these points to the eyes of 

 the Annelidansf, and from the very obvious manner in which the 

 worms evince their sensibility to the impressions of light. The spe- 

 cies have no other visible exterior organs : they move in an even 

 continuous manner by undulations, frequently imperceptible, propa- 

 gated along the body, which they often throw into knots and stric- 

 tures, or extenuate to that degree, that the ordinary breadth shall be 

 more than four times its diameter when in this state of extension. 



The internal structure is most easily seen when the worm is 

 slightly pressed between plates of glass, and placed under the micro- 

 scope. It is more beautiful and complex than the plainness and 

 simplicity of the exterior would lead us to believe, nor indeed can I 

 unravel or describe it with the accuracy that is desirable. The mouth 

 is difficult to be detected, and its position and form are generally in- 

 distinct ; but sometimes it becomes evident enough, and in other 

 cases, its exact place is often shown by a slight sinus or emargination 

 in the anterior extremity in which it is placed : it is a simple circular 

 pore without any hard parts (PI. II. B. fig. 5 m). From it an 

 intestine descends down the centre of the body, in a straight or un- 

 dulating line, according to accidental circumstances of position, to 

 the opposite extremity, where it opens outwardly by a pore similar 

 to the mouth, and equally simple. The intestine (PI. II. A. fig. 5 i, i) 

 is a cylindrical tube of a firmish texture, and of nearly equal calibre 

 throughout, though subject to partial and temporary constrictions 

 and dilatations, and, as just remarked, its course can be made straight 

 or tortuous at the will of the animal, evidently to accommodate it to 

 the length of the body in its extreme variations ; for being apparently 

 of a much less contractile tissue than the body itself, the intestine is 

 doubled in sinuous folds when the worm contracts and shortens ; but 

 when this draws itself out in a long line, the intestine becomes a 

 straight canal, with something of the character and office of a verte- 

 bral column. In tracing the intestine from the mouth downwards, 

 the structure appears to be homogeneous, and alike throughout in 

 the species placed in the second section of the genus, but in the true 

 Nemertes we meet with some remarkable peculiarities towards the 

 middle of its course. First, we perceive on each side a small circular 

 spot or cavity, in each of which are three spines (figs. 1, 2, 4) with 

 their sharp points directed outwards ; beneath these there is a cup- 

 shaped organ (fig. 3) encircled above with a faintly plaited membrane, 

 and armed in the centre with a strong spine, which can be compared 

 to nothing more aptly than to a cobler's awl in miniature, the part 

 representing the handle being very dark, and the point transparent 



* Hist. nat. des Anim. s. Vert. iii. 177. 



t See Miiller " Sur les Yeux probleraatiques des Annelides," in the Ann. des 

 Sci. nat. xxii. 19 ; and Grant's Outlines of Comparative Anatomy, p. 252-3. 



