THE LAMELLIBRANCH1A 243 



The testis of a male or hermaphrodite is always readily recog- 

 nisable by its brilliant white colour ; on the other hand, the ovary 

 is often red (Mactra, Donax, etc.). The ovum is derived from a cell 

 of the ovarian epithelium, but in most cases the neighbouring cells 

 contribute to the formation of its vitellus (Cyclas, etc.). The ovum 

 is surrounded by a vitelline membrane, which is often fairly thick 

 (Unionidae, Anatinacea, etc.), and is only interrupted at the micro- 

 pyle, or point of attachment to the ovarian wall. It is at this point 

 that the spermatozoon effects an entrance. The vitelline membrane 

 disappears after the first stages of segmentation, except in incubatory 

 forms. A true ovarian follicle, formed of a continuous and regular 

 envelope of epithelial cells, has been described only in Pseudokellya- 

 (Fig. 220, 



&.CL 



FIG. 221. 



Pseudokellya cardifonnis, Smith, left-side view (the left pallial lobe removed), ad', ad" r 

 anterior and posterior adductor muscles; br, br 1 , internal and external 'gill -plates ; em, 

 embryos in the internal gill-plate ; o.a, anal orifice of the mantle ; 0.6, branchial oriflce of the- 

 mantle ; o.p, limits of the pedal orifice of the mantle ; p, foot ; pal, labial palp ; rep, rep' t 

 anterior and posterior foot-retractor. 



III. EMBRYOLOGY. 



Viviparous Lamellibranchs are unknown, but some few appear 

 to be viviparous because they are incubatory. This is the case in 

 some Filibranchia (Area vivipara and Philobrya) and in many Eula- 

 mellibranchia, principally in the Submytilacea (certain species of 

 Ostraea, Condylocardia, Lasaea, Bornia, Scioberetia, Entovalva, Thecalia, 

 Unionidae, Cyrenidae, Pseudokellya, Teredo, etc.). The ova then 

 are hatched after their escape from the genital organs, but in the 

 greater number of incubatory forms they are retained for a 

 certain time, in some cases up to the time of hatching, in the inter- 

 lamellar branchial spaces. In certain Unionidae (Castalina, Arconaia, 

 Pseudodon, etc.) and in Lasaea (Fig. 222) and Pseudokellya (Fig. 

 221, em) they are retained in the internal interlamellar space, as 



