LAMELLIBRANCHIA TA. 291 



into the body. In these caeca the generative products both male and female 

 <ievelope side by side. Each duct opens anteriorly into a recess common to it and 

 the duct of the organ of Bojanus or nephridium of the same side. 



The nephridium consists of long caeca which give origin to other caeca and 

 extend even into the mantle. They lie in the body superficially to the genitalia. 

 The renal channels open into a long chamber. This chamber or duct communicates 

 with the recess above mentioned and by a reno-pericardiac canal with the peri- 

 cardium. The nephridium of the Oyster differs strikingly from that of other 

 Lamellibranchiata (Fig. 4) in not being compact. Some parts of the renal channels 

 are covered with a ciliated, other parts with a stratified epithelium. 



Hoek, Tijdschrift der Nederlandsche Dierkundige Vereeniging, Suppl. i. 

 Leiden, 1883, (in French and Dutch). 



A resume of anatomy of the renal and genital organs. Abstract in Journal 

 of Roy. Micr. Soc. (2), iii. 1883, pt. i, p. 354. 



Nervous system. Duvernoy, Me"moires de ITnstitut, xxiv. 1854, Monograph, 

 p. 6 1. 



Visual organs of O. Virginica. Sharp, Mitth. Zool. Stat. Naples, v. 1884. 



Adductor muscle. Plateau, A. Z. Expt. (2), ii. 1884. 



Sexes of oysters, Ryder, A. N. H. (5), xii. 1883. O. Virginica and O. angu- 

 lata have the sexes separate; Bouchon-Brandely, C. R. xcv. 1882. 



Development. Horst, A. N. H. (5), ix. 1882 ; Brooks, Johns Hopkins Univ. 

 Biological Studies, Report, Chesapeake Zool. Lab., Session 1878; Ryder, Report 

 of U. S. Commission of Fish and Fisheries for 1882 (published 1884). 



Green colour in oysters. Puysegur, cf. Journal Roy. Micr. Soc. iii. 1880; 

 Ryder, ibid. (2), iii. 1883 ; both in full in Reports of U. S. Commission of Fish and 

 Fisheries for 1882 (published 1884). Ray Lankester, Q. J. M. xxvi. 1886. 



FIG. 2. Digestive tract of the same, in the position it would occupy in a specimen placed like 

 the one figured above (Fig. i). After Brandt and Ratzeburg, Medizin. Zoologie, Berlin, ii. 1833, 

 PL XXXVI. Fig. 4. 



a. The two pairs of labial tentacles. 



b. Anal termination of the intestine, lying to the dorsal side of the adductor 



muscle,/. Fig. i. The anus is funnel-shaped. 



c. Loop of intestine occupying the ' oral process ' of Hoek, which projects 



in front of the visceral ganglia (g, Fig. i). 



d. Stomach, upon the side of which, turned towards the dissector, lies the 



coil of intestine immediately preceding the anus. 



FIG. 3. Heart and principal vessels of Area Noc. From Poli, Testacea utriusque Siciliae, 

 ii, Parma, 1795, PI. XXV. Fig. 2. 



The heart is aberrant, inasmuch as there are not only two auricles as 

 in other Lamellibranchiata, but the ventricle also is double. 



a. Branchial venous, i. e. efferent vessels. The shaded vessel below repre- 



sents the branchial arterial, i. e. afferent vessels. 



b. Mantle vessels, entering with a. 



U 2 



