§^ 186, 187. 



THE ACEPHALA. 



201 



§ 186. 



As yet, organs of hearing with the Acephala have been found only among 

 the Lamellibranchia. They are here feebly developed, consisting only of 

 two simple round capsules filled with a transpai-ent liquid. Their very thick 

 and somewhat solid walls are homogeneous and transparent ; they enclose a 

 vitreous spherical otolite, of a crystalline structure,*^' and composed of car- 

 bonate of lime. These otolites constantly keep up very singular swing- 

 ing and rotatory motions, which instantly cease, however, when the capsule is 

 ruptured.'-' These auditory capsules when present, are situated in the foot 

 in front of the pedal ganglia with which they always communicate,'^' either 

 contiguously, or by two auditory nerves which they receive.'*^ 



§ 187. 



Organs of vision are very common with the Acephala, and always many 

 in number. With some, they occupy a large portion of the borders of the 

 mantle ; with others, they are confined to the external orifices of the longer 

 or shorter mantle-tubes:'" 



1 These organs were first noticed by me with the 

 Naiades, Cardiacea, and the Pyloridae ; but were 

 i-egarded as of a doubtful nature. Since then, after 

 comparing them with the auditory organs of the 

 embryos of fish, I am satisfied that they are really 

 very simple oi-gans of hearing ; see Miiller's Arch. 

 1S3S, p. 49, and PFiegmann's Arch. 1841, 1, p. 148, 

 Taf. VI. fig. 1, 2 {Cyclas cornea) ; or, the Ann. d. 

 Sc. Nat. X. 1838, p. 319, XIX. 1843, p. 193, PI. 

 II. B. It appears, moreover, that similar corjjus- 

 cles are found in other orders of these animals. 

 Thus, Delle Chiaje mentions with Salpa neapoli- 

 tana, an organ situated above the nervous centre 

 which exactly resembles the auditory capsules I 

 have discovered in the foot of Cyclas. Unfortu- 

 nately he has neither figwed nor carefully described 

 this organ (Descriz. &c. III. p. 45, Tav. LXXVI. 

 fig. 1, 1.). Eschricht (Anat. Beskriv. af Chelyo- 

 soma Macleayanum, p. 9, fig. 4, 6, d. y. and fig. 5) 

 has also regarded as an auditory organ a remarka- 

 lile apparatus which he found near the nervous cen- 

 tre of a simple Ascidian. This consists of a jiyri- 

 form vesicle filled with whitish matter, and of a 

 clavate body which has, upon its large end, a fis 

 sure and two lateral depressions. 



Delle Chiaje^s figure (Descriz. &c. III. Tav. 

 LXXXII. fig. 4.), ot the principal ganglionic mass 

 of Cynthia papillata, reminds me of the cla- 

 vate body of Chelyosoma and leads me to think 

 that this author has confounded it with the nervous 

 centre. I think that this organ exists generally 

 with both the simple and compound Ascidiae, for 

 Sarii:ny lias noticed witli Cynthia, Pkallusia, 

 Aplidiiim, Polyclinum, Botryllus, Eucoeliian, 

 Synoecium, Pyrosoma, &c., two tubercles near the 

 nervous ring which surrounds the respiratory tube 

 (Tuhercule ojit^rieur et po.iterieur). And, to 

 judge from his figure (Mem. &c. PI. VI. fig. 1-, 2, 

 4 , h. PI. VII. fig. 2'), of one of these tubercles, with 

 Cynthia^ these organs appear analogous to the cla- 

 vate body just mentioned. At all events, these 

 tubercles deserve, with Zootomists, more attention 

 than has lutherto been given them. 



2 These motions are probably due to th« ciliated 

 cpitheUum lining the cavity of the capsuo s ; see, 

 below, the auditory organs of the Gasteropoda 



3 I have been un.able as yet to find these capsules 

 with the apodal Lamellibranchia, — at least, with 

 IHchogonia, and Mytilus. They appear to exist, 

 however, for recently Dexliayes lias found them both 

 iu Teredo. Here they were situated iit the extrem- 



ity of the septa lying betweea>the pericardium and 

 the elevator of the anus, and upon which the ante- 

 rior extremity of the branchiae is inserted ; see 

 Comp. rend. 1846, X.VII. No. 7 ; or Froriep's 

 neue Not. No. 813, p. 323. 



4 With Cyclas, and Tellina, the auditory cap- 

 sules are contiguous with the ganglia of the Par 

 pedale. With Anodonta, Unio, Cardium, and 

 Mya, they are a little removed. 



It.is remarkable that these organs appear very 

 early in the embryos of certain Limiellibranchia 

 (Cyclas), while in others {Anodonta and Unio), 

 no trace of them is seen during the embryonic 

 life. 



1 Poli (loc. cit. II. p. 153, 107, Tab. XXII. fig. 

 1,4; and Tab. X-WII. fig. 5, 14,16), was the 

 first to compare to human eyes these remarkable 

 bodies, which, brilliant as diamonds, lie upon the 

 borders of the mantle of Pecten and Spondylua, 

 with this expression : Ocelli smarasdino colore 

 coruscantes. Nevertheless, it is only of late that 

 these organs have received much attention. Gar- 

 ner (On the Anat. of the Lamellibr. Conchifer. &c. 

 PI. XIX. fig. 1, c. 3) was the first to notice anew 

 the Ocelli of Pecten. Grant (Outlines, &c., p. 

 258) haii described those of Pecten 3,nd Spondylu-f 

 as organs long known. Grube (Muller's .\rch. 

 1840, p. 24, Taf. III. fig. 1, 2), and AroAn (Ibid, 

 p. 381, Taf. IX. fig. 16) have described the struc- 

 ture of these organs, and, quite recently. If^ill (Fro- 

 riep's neue Not. 1844, No. 622, 623) has treated 

 this subject most profoundly. 



Deshayes is not satisfied of the existence of or- 

 gans of visicjii with the Pectinea, while Dnvernoy 

 regards as such the bodies situated on the border 

 of theii- mantle (Instit. 1845, p. 52, 88). It is astonish- 

 ing that Deshayes should have denied eyes to tli:; 

 Pectinea, where they are so complete. He could liave 

 better denied tbem to Phallusia, Area, Ostrea,im\ 

 other Acephala. During ray last visit at Venice 

 and at Trieste, I examined living individuals of the 

 genera Area, Ostrea, Pinna, as well as other Ijiun- 

 ellibranchia and various Ascidiae ; but with all 

 possible care, I was unable to verify IVilfs descrip- 

 tion (loc. cit.) of the eyes of these animals. In 

 most cases, the bodies which he has desicribed iia 

 eyes, have appeared to nie only as simiilo excres- 

 cences of the mantle, which arc variously colored, 

 but are wholly without the indispensable optic ap- 

 pai-atus for a visual organ. 



