THE ELAMELLIBRANCAIA 24 
LoS) 
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, Donaz, 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, ete.). 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, fol). 
Fig. 221. 
Pseudokellya cardiformis, Smith, left-side view (the left pallial lobe removed), ad’, ad’, 
anterior and posterior adductor muscles; br, br’, internal and external ‘gill-plates; em, 
embryos in the internal gill-plate ; 0.a, anal orifice of the mantle ; 0.b, branchial orifice of the 
mantle; o.p, limits of the pedal orifice of the mantle; p, foot; pal, labial palp; rep, rep’, 
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 (Arca 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, ete.) and in Lusaea (Fig. 222) and Pseudokellya (Fig. 
221, em) they are retained in the internal interlamellar space, as 
