LAMELLIBRANCHIATA. 489 



in the region between. With the development of interlamellar and inter- 

 filamentar junctions the blood-channels cease to run for the most part 

 in the filaments but follow the course of the junctions. At the same time 

 the branchial cavity is divided into an infra- and a supra-branchial cavity 

 by the union of the recurved portions of the inner filaments as described 

 above, cf. p. 130, ante. The anus lies at the posterior extremity of the supra- 

 branchial cavity, which is continued on into the exhalent siphon when 

 present, the cavity of the inhalent passing into the infra-branchial chamber. 

 The nephridia are always paired, and bent or folded upon themselves. 

 The ducts open externally by well-marked pores, usually placed as in 

 Anodonta, (p. 134, cf. PI. vi.), and the two ducts usually communicate one 

 with another near the external opening. The glandular portion of each 

 organ generally communicates with the pericardium, but the ciliary currents 

 are said to set outwards as a rule. The Oyster differs from other Lamelli- 

 branchiata in the structure of the nephridia, see p. 291. The genitalia 

 are paired branched glands terminating in round or cylindrical caeca in 

 which the genital products are formed. The branches are intermingled with 

 the branches of the liver, but may be partially (Anomid) or wholly con- 

 tained in the mantle (Mytilus}. The ducts open as a rule near the neph- 

 ridial openings, or into a groove common to the two sets of apertures 

 (Ostrea, Area, Pinna, &c.), which may be converted (?) into a canal with 

 a small opening (Pecten, Lima, Spondylus). The sexes are separate with 

 few exceptions such as Ostrea cdulis, Cardinm serratum, in which male 

 and female products are developed in the same caeca but at different times ; 

 or Pecten, Aspergillum, &c., in which the male and female parts of the 

 gland are more or less separate, but both genital products ripen at the 

 same time and the animals may therefore be self-impregnating. Cyclas 

 and Pisidium are also hermaphrodite, but it is not certain to which group 

 of the two described they belong. The spermatozoa find their way to 

 the ova either in the water, or, what appears to be generally the case, 

 in the mantle cavity, or in that of the outer gill. In Kellia, Galeomma 

 and Montacuta bidentata the ova develope within the ovary, but it is not 

 certain whether the animals are hermaphrodite and self-impregnating, or 

 the spermatozoa pass up the genital duct. The ovum is enveloped in 

 a membrane, which is sometimes (Anodon, Unio) a vitelline membrane, or 

 is formed by the superficial hardening of an albuminous layer (Scrobi- 

 cularid). It is attached to the wall of the ovary by a protoplasmic stalk. 

 Hence the membrane is incomplete at this spot, which forms the micropyle 

 when the ovum is detached. The ovum always remains connected with 

 the envelope at the micropyle, but is separated elsewhere by an albuminous 

 layer. Segmentation is unequal. The gastrula is either invaginate or 

 formed by overgrowth. The velum of the Veliger is a circular ridge, and 

 from the centre of the velar area arises a long flagellum. This is wanting in 



