138 DESCRIPTIONS OF PREPARATIONS. 



rounded by plasma-cells (p. 115), and consists of a fibrillated connective tissue 

 coat, within which is a nervous layer (?) formed by the auditory nerve, and most 

 internally a layer of ciliated epithelium. Whether special auditory hairs are 

 present, as in Cyclas and some Gastropoda, is uncertain. The cavity of the 

 vesicle contains a fluid in which floats a spherical calcareous otolith, single as 

 in all Lamellibranchiata. The nerve is derived from the cerebro-pedal connective, 

 according to Simroth, not from the pedal ganglion, as ordinarily stated. See 

 Z. W. Z. xxvi. p. 270, PI. xvii. fig. 56; and for otocyst, PI. xviii. figs. 62 and 68. 

 It is not certain if the auditory nerve is similarly derived in other Bivalves. It 

 is in other Mollusca. The only specimens of Anodonta at my command were 

 not sufficiently fresh to enable me to decide whether or no the nerve-supply 

 is invariably derived as Simroth describes. It is possible that in some cases 

 the nerve passes through the pedal ganglion, but without being derived from it. 



Nervous system. Duvernoy, Mdmoires de 1'Acade'mie des Sciences de 

 1'Institut, xxiv. 1854, p. 87 (with plates referred to). Histology. Vignal, A. Z. 

 Expt. (2) i. 1883. Homologies of, and osphradium. Spengel, Z. W. Z. xxxv. 1881, 



P- 373- 



Tactile cells. Flemming, A. M. A. v. 1869; vi. 1870, p. 453; cf. ibid, xxiii. 

 1883. 



Otolithic vesicle. Leydig, Lehrbuch der Histologie, 1857, p. 278, with references, 

 p. 283. Simroth, Z. W. Z. xxvi. 1876. 



Generative organs in Lamellibranchiata. De Lacaze-Duthiers, A. Sc. N. (4) 

 ii. 1854. Of Mytilus. MTntosh, A. N. H. (5) xv. 1885. Occasional herma- 

 phroditism in Anodonta. De Lacaze-Duthiers, A. Sc. N (4) iv. 1855. 



Passage of ova to gills. Von Baer, Meckel's Archiv. (Archiv. f. Anat. und 

 Physiol.), 1830. Possible passage of ova from one Mussel to another. Von Hessling, 

 Z. W. Z. x. 1859-60, p. 358. The different parts that act as Marsupia. Bronn, 

 Klass. &c. iii. i, p. 442. 



Development with literature. Balfour, Comparative Embryology, i. p. 220. 

 Schierholz, Zur Entwickelungsgeschichte der Teich- und Fluss-muschel, Berlin, 

 1878 ; cf. Id. Z. W. Z. xxx. 1878. Post-embryonal development of Najaden 

 (Anodonta}. Schmidt, A. N. 51. Cyclas cornea. Ziegler, Z. W. Z. xli. 1885. 



28. COMMON COCKROACH (Periplaneta orientalis], FEMALE, 



Dissected to show its digestive, renal, nervous, and reproductive systems. 



AMONG external features characteristic of the class Insecta, the head 

 with the antennae, the three pairs of jointed thoracic limbs, and the seg- 

 mented abdomen may be noted. The short tegmen or wing-cover of the 

 female of this species is visible above the second limb on the right side, and 

 posteriorly, on the same side also, at the extremity of the abdomen, one 

 of the two short jointed cerci anales found in many Orthoptera, &c. 



The dorsal body walls or terga, and the fat body which abounds be- 

 tween the viscera, especially in the abdomen of these insects, even in their 



