162 



THE ANNELIDES. 



^151. 



enveloped wi'th a layer of black pigment/^ Eacli of these bodies receives 

 a nervous filament from the 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 Aniphinoniae, Nereideae, Euniceae, 

 and Aphroditae, have two to four very distinct eyes.*'' In these, there is 

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

 often has, above, a very distinct round pupillary 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 oif usually from the upper surface of the brain have, after 

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

 ment. It is said that with some the light-refracting body and the pupillary 

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

 from darkness.'**' 



///. 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 Vestibula, containing many otolites.*'* 



5 Jfeber was the first to show that the black 

 «pecks of Sansuisuga officinalis were really eyes 

 iMeckeCg Arch. 1827, p. 301, Taf. III. fig. 24). 

 This has been confirmed by BTandt (Med. Zool. I. 

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

 cently, Wagner 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 Wagner, Lehrbuch, d. ver- 

 gleich. Anat. 1835, p. 428 ; also Lehrb. d. speziellen 

 Vhys. 1843, p. 383, and, Icon, physiol. 1839, Tab. 



XXVIII. fig. 16. 



6 Brandt has been able to trace the ten optic 

 nerves of Sanguisuga officinalis from the brain 

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



XXIX. B. fig. 2).* 



^ With Glycera, Aricia, Arenicola., and Cir- 

 ratulus, the eyes are wanting. With Goniada, 

 and Nephtys, there are only simple jjigment specks 

 upon the head. With Eunice, Phyllodoce, and 

 Alciopa. there are two eyes ; and four with Nereis, 

 Hyllis, Hesione, and Amphinome. The genus Al- 

 ciopa is well suit'.d, from its large size, for the dissec- 

 tion of these organs. 



i For a most detailed description of the eyes of 

 Her CIS. we are indebted to Müller (Ann. d. So. 

 JSat. XXII. 1831, p. 22, PI. IV. fig. 6 10), and Wag- 



ner (Jjehrh. A. Physiol, p. 383, and Icon, physiol 

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

 (Zur vergleich. Physiol, d. Blutes, 1833, p. 55), 

 could not, any more than Müller, 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. According to Rathki (De Bopyro et 

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

 reis pulsatoria and lohulata want the pupil, 

 although it is present with those of Nereis Dumer- 

 ilii. According to Wagner, the pupil is wanting 

 with the two posterior eyes, but is present with the 

 two anterior ones, with most of the Nereideae. 



1 I have compared the swellings noticed upon the 

 oesophageal ring of Arenicola, by Grube and Stan- 

 nius (see Wiegm<inn's Arch. 1841, I. 166), to the 

 auditory vesicles of MoUusks, and their contents to 

 otolites ; since then, Quatrefages has recognized 

 the presence of similar auditory organs containing 

 many otolites with two species allied to Amphicora 

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

 Nat. 1844, 11. p. 94). Frey and Leuckart (Beitr. 

 &c. p. 81), after a vei'y 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 Hirudinei, see Moquin-Tandon, 

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

 .ing to him, they contain neither a lens nor a vitre- 

 ous humor, and are only light-perceiving organs. 

 ■See also Leydig (loc. cit. p. 129) who makes the 

 ■^uUowing statement upon the nature of these bodies 



with 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." — iJD. 



