162 



THE ANNELIDES. 



<§>! 



51. 



enveloped with a layer of black pigment.'''^ Each of these bodies receives 

 a nervous filament from tlie cerebral ganglion. Undoubtedly, these fila- 

 ments are optic nerves, and the cylindrical bodies are light-refracting and 

 light-concentrating organs.'"^ 



Many of the Dorsibranchiati are entirely without eyes, having only the 

 eye-specks ; but others, belonging to the Amphinoinae, Nerei'deae, Euniceae, 

 and Ajjhroditae, have two to four very distinct cyes.<"> In these, there i.s 

 an eye-ball invested with a black or brown pigment layer : and this layer 

 often has, above, a very distinct round pupilUiry opening, covered by the 

 skin, which bulges out like a cornea. At the central portion of this 

 layer, there is concealed a transparent body, which is very probably sur- 

 rounded by a retina-like expansion of the optic nerve. The optic nerves 

 ■which are given oflF usually from the upper surface of the brain have, after 

 a short course, and before entering the pigment layer of theeye,.an enlarge- 

 ment. It is said that with some the light-refracting body and the ])Upillary 

 opening are wanting. In such cases, the eyes could only distinguish light 

 from darkness. *^^ 



III. Organs of Hearing. 



§ 151. 



Although it has never been doubted that the Annelides can perceive 

 sounds, yet it is of late only that the attention has been directed to the 

 locality of the auditory organs. The two vesicles, which, with some Chae- 

 topodes, are situated near the oesophageal ring, and contain crystalline 

 bodies, may be regarded as simple Vesiibula, containing many otolites."* 



6 IFel/er was the first to show that the black 

 gpccks of Snmruifnirra olftcinalin were really eves 

 (Meckel's Arch. 1827, ]). oOl, Tuf. III. li-. 21). 

 This has been confirmed by Brandt (Med. Z )ol. I. 

 p. 251, Taf. XXIX. A. fig. 10-12), and more re- 

 cently, WagncT has discovered in the interior of 

 the pigment layer, a transparent body, composed, 

 he thinks, of two parts, a crystalline lens and a vit- 

 reous portion ; see IVagner^ Lehrbuch, d. ver- 

 gleich. .\nat. 1835, p. 428 ; also Lehrl). d. speziellen 

 Phys. 1S43, p. 383, and, Icon, physiol. 183U, Tab. 



XXVIII. fig. IG. 



fi Brandt has been able to trac^ the ten optic 

 nerves of San^uiauga officinalis from the brain 

 even to the eyes (Med. Zool. loc. cit. p. 250, Taf. 



XXIX. B. fig. 2).* 



J' Witli O/i/cera, Aricia, Arenicola, and Cir- 

 ratiiliis, the eyes are wanting. With Goniada, 

 and Nr/thti/s, there are only simple i)igments|)ecks 

 apon the liead. With Eunice, PIn/llodorp, and 

 Alciojja, tli;'re are two eyes ; and four with Nereis, 

 Sy/lis, Hesione, and Amphinome. The genus Al- 

 eiopa is well suit d, from its large size, for the dissec- 

 tion of these organs. 



A For a most detailed descripti in of the eyes of 

 Herds, we are indebted to Muller (Ann. d. Sc. 

 Nat. XXII. 1831, p. 22, PI. IV. fig. 6 10), and «'a^- 



ner (Lehrb. d. Physiol, p. 383, and Icon, physiol. 

 Tab. XXVIII, fig. 15). fVagner, who, formerly 

 (Znr vergleich. Physiol, d. Ulutes, 1833, p. 55), 

 could not, any more than Muller, perceive the 

 light-refracting body, has at last seen it distinctly. 

 For my own part, I can confirm its presence in the 

 two eyes of Eunice gigantea, which have a circu- 

 lar pupil. At^cording to Rat/iki (De IJopyro et 

 Nereide, p. 44, Tab. II. fig. 4, 5) the eyes of Ne- 

 reis pulsotoria and Inhu/ata want the pupil. 

 although it is present with those of Nereis Dumer- 

 ilii. According to tVagner, the pujiil is wanting 

 with the two i)osterior eyes, but is present with the 

 two anterior ones, with most of the Neriideae. 



1 I have c jmiiared the swellings noticed upon the 

 oesophageal ring of Arenicola, liy Grube and Stan- 

 nius (see IViegmann\i Arch. 1841, I. 166), to the 

 auditory vesicles of Mollusks, and their contents to 

 otolites ; since then, Quatrefagcs has recognized 

 the presence of similar auditory organs containing 

 many otolites with two species allied to Amphicora 

 (Compt. rend. XI.X. 1844, p. 195, and Ann. d. sc. 

 Nat. 1844, II. p. 94). Frcy and Leuckart (Beitr. 

 &c. p. 81), after a very careful examination of the 

 organs of Arenicola, which I have regarded as au- 

 ditive, have confirmed this opinion. 



* [ § 150, note 6.] For further details on the ocu- 

 lar organs of the Ilirudinei, see Moquin-Tandon, 

 loc. cit. Ed. 1846, p. 80, PI. VIII. fig. 11. Accord- 

 ing to him, they contain neither a 'ona nor a vitre- 

 ous humor, and are only light-perceiving organs. 

 Bee alsa Leydi;; (loc. cit. p. 129) who makes the 

 following statement upon the nature of these bodies 



vi\i\\ Piscicola ; "They receive no nerve, neither 

 do they contain a light-refracting body. I regard 

 them as simple ornaments, wholly analogous to the 

 corresponding pigment dots on the pedal shield, 

 with which they also correspond in color and dis- 

 tribution." — Ed. 



