vii MOLLUSC A GENITAL ORGANS 229 



with a few hermaphrodite genera : Cyclns, Pisidium, Entovalva ; (3) genera (Ostrcea, 

 Pccten, Cardium) with a few hermaphrodite species ; (4) occasional cases of herma- 

 phroditism in species the sexes of which are usually separate : Anodonta. The 

 hermaphroditism of the Lamellibranchia is, however, always incomplete in the sense 

 that the spermatozoa and the eggs do not ripen simultaneously. 



In the Anatinacea and Septibranehia, there are on each side entirely separate male 

 and female gonads, whereas all other hermaphrodite Lamellibranchs have a her- 

 maphrodite gland on each side. 



(5) Cephalopoda. The sexes are always separate in this class. It has already 

 been mentioned that the germinal sacs form a part of the secondary body cavity, witli 

 which they are in open communication. 



One single unpaired gonad is always found, lying in the uppermost part of the 

 visceral dome. It is a variously-formed sac (peritoneal sac or genital capsule), lined 

 on all sides by an epithelium often to a great extent ciliated, which is in reality the 

 peritoneal epithelium of the secondary body cavity. The whole of the epithelium 

 covering the wall of the gouad is not, however, germinal, but only that on its anterior 

 side (that turned to the shell). The germinal layer here forms what may be called, in 

 the narrower sense, the ovary or the testis, which is then said to be contained in a 

 peritoneal sac or an ovarial or testicular capsule, or else to project into or be suspended 

 in such sac or capsule. The whole apparatus is really a gonad, in which the places 

 of formation of the reproductive cells are localised on the anterior wall. 



From this it is clear why the testes and ovaries do not appear to possess efferent 

 ducts of their own, but to empty their products into their respective capsules, these 

 products passing out into the mantle cavity through the ducts of these capsules 

 (oviducts and seminal ducts). Since, however, the entire germinal sac corresponds 

 with the genital gland of a Gastropod or a Lamellibranch, the reproductive pro- 

 ducts in reality merely fall into the cavity of this gland (the testicular and ovarial 

 capsules), and pass out through the ovarial and seminal diicts, which exactly corre- 

 spond with the same ducts in the Gastropoda, Lamellibranchia, and Chitanidce. 



The genital cavity has also another means of communication with the exterior, 

 since, in the Cephalopoda, it is in open communication with the remaining part of 

 the secondary body cavity, whether the latter forms a viscero-pericardial cavity (Deca- 

 poda) or is reduced to the "water canal system " (Octopoda). This latter part of the 

 body cavity again is connected, by means of the nephridia, with the mantle cavity. 



In this way, the genital cavity communicates with the mantle cavity directly by 

 means of the oviduct or seminal duct, and indirectly through (1) the viscero-pericardial 

 cavity or the " water canal system," and (2) the nephridia. This second way of 

 communication, however, is never used for discharging the genital products. 



The female germinal layer or ovarial layer (the ovary in the narrower sense) is 

 always found on the anterior wall of the gonad, and varies considerably in^structure (Fig. 

 188). We can always distinguish (1) the eggs, and (2) the ovigerous wall. The former 

 are stalked, and project from the wall into the cavity of the gonad (the cavity of the 

 ovarial capsule). The largest and oldest eggs are covered by a follicular epithelium, 

 and this latter by the general epithelium of the wall of the gonad, which also covers 

 the stalk. Eacli egg has a separate stalk. The youngest eggs are mere prominences 

 on the wall, which in the process of growth acquire a stalk, by means of which they 

 remain connected with the wall from which they project. This arrangement is 

 exactly like that in the Chiton. When the eggs are mature, the follicle bursts, they 

 fall into the genital cavity, and thence reach the exterior through the oviduct. 



In Nautilus (Fig. 188, A) and Eledoiie the whole wall of the gonad, with the 

 exception of the posterior surface, can produce eggs ; these stand out from it all over 

 on simple stalks. In Argonauta (Fig. 188, B] and Trcmoctopus also, the whole ova- 

 rial capsule except the posterior wall produces eggs, but the egg-bearing region (to 



