<§>^ 186, 187. 



THE ACEPHALA. 



201 



§ 186. 



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

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

 two simple round capsules filled with a transparent 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 

 regarded 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 organs of hearing ; see Mut/er's Arch. 

 1838, p. 49, and fVie^mann's Arch. 1841, 1, p. 148, 

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

 Sc. Nat. X. 1838, p. 319, XI.X. 1843, p. 193, PL 

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

 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 figured nor carefully described 

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

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

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

 has also regarded as an auditory organ a remarka- 

 ble apparatus which he found near the nervous cen- 

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

 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\<i figure (Descriz. &c. III. Tav. 

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

 of Cynthia papitlata, 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 

 Savi^ny has noticed with Cyntkia, Pha/lusia, 

 Aplidium, Polyelinum, Botryllus, Eucoelium, 

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

 nervous ring which surrounds the respiratory tube 

 (Jl'ubercule ant&riexir et posterieur). And, to 

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

 4 , h. PI. VII. fig. 21), 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 hitherto been given them. 



2 These motions are probably due to thp ciliated 

 epitheUum lining the cavity of the capsui. s ; see, 

 below, the auditory organs of the Gasteropoda 



3 I have been unable as yet to find these capsules 

 with the apodal Lamellibranehia, — at least, with 

 Tickosonia, and Mytilus. They appear to exist, 

 however, for recently Deshayes has found them both 

 iii Teredo. Here they were situatefl at, the extrem- 



ity of the septa lyhig between the pericardium and 

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

 rior extremity of tlie branchiae is inserted ; see 

 Comp. rend. 1846, XXII. No. 7 ; or Froriep^t 

 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 Lamellibranehia 

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

 no trace of them is seen during the embryonic 

 Ufe. 



1 Poli (If'c. cit. II. p. 153, 107, Tab. X.XII. fig. 

 1, 4 ; and Tab. XXVII. fig. 5, 14, 15), was the 

 first to compare to human eyes these remarkable 

 bodies, which, brilliant as diamonds, lie ujjon the 

 borders of the mantle of Pecten and Spondylus, 

 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, e. 3) was the first to notice anew 

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

 258) has described those of Pecten and Spondylus 

 as organs long known. Grube (Muller's Arch. 

 1840, p. 24, Taf. lU. fig. 1, 2), and Krohn (Ibid, 

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

 ture of these organs, and, quite recently, IVill (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 vision with the Pectinea, while Duvemoy 

 regards as such the bodies situated on the border 

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

 ing that Deshayes should have denied ej^es to tiie 

 Pectinea, where they are so complete. He could have 

 better denied them to Phallusia, Area, Ostrea, and 

 other Acephala. During my last visit at Venice 

 and at Trieste, I examined living individuals of the 

 genera .4rca, Ostrea, Pinna, as well as other I^uii- 

 ellibranchia and various Ascidiae ; but with all 

 possible care, I was unable to verify frill's descrii>- 

 tion (loc. cit.) of the eyes of these animals, lii 

 most cases, the bodies which he has described as 

 eyes, have appeared to me only as simple e.xcres- 

 cences of the mantle, which are variously colored, 

 but are wholly without the indispensable optic ap- 

 paratus for a visual organ. 



