llORltllLL— ANATOMY OF PLACUNA 87 



munication of these large lobes of the gonad is a very noticeable feature in 

 Placuna. 



Except the unimportant dorsal or hinge lobes which belong strictly to the main 

 mass of the gonad enveloping the stomach and liver and are median in position, all the 

 above-mentioned branch masses lie within the right mantle. The only portion that 

 penetrates the left mantle is a small branch from the pyloric csecal lobe which crosses 

 from the right side at the attachment of Ihe ventral end of the kidneys to expand into 

 a disc-shaped mass covering and surrounding the proximal ends of the pallial muscles 

 of this region. 



The gonad in the male appears usually to be rather smaller than in the femnle. 

 The sexes may be distinguished by the colour of the gonad ; in the ripe female it is 

 normally of an orange yellow tint, in the male it is duller and has more of a soiled 

 yellow hue. 



The gonad opens into the internephridial passage by a small aperture {Go.a., 

 fig. 6) situated iu the roof of the passage. The aperture is therefore anterior to the 

 ventricle, and midway between the auricles. From the position of the single genital 

 aperture being in the roof of the internephridial passage, the reproductive products, 

 when emitted into the renal organ, will pass readily in part to the right and in part to 

 the left, and so be conducted into the respective ventral limbs of the nephridium, 

 whence emission to the exterior will take place through the renal apertures situated on 

 either side of the parieto-splanchnic ganglion. 



Microscopical examination shows that intermingled throughout the whole gonad 

 in both males and females is a very peculiar soft spongy tissue of obscure significance. 

 Its nature is absolutely distinct from that of the reproductive tissue proper ; in 

 appearance it simulates a delicate and loose network of connective tissue. In young 

 specimens the proportion of this enigmatical tissue is very large and it would appear to 

 perform some distinct function other than that of a mere stromal framework — possibly 

 it functions as a food reserve (see p. 91 for further remarks on the significance of 

 this tissue). 



Histologically each gonad is made up of a mixture of the spongy tissue above 

 mentioned with great numbers of ramifying tubules on which cluster, grape-like, dense 

 masses of saccate alveoli lined with germinal epithelium. The ultimate structure of 

 the alveoli is best seen in a female. Here they are distinguishable as wide caeca 

 containing ova in various stages of development, from a tiny cell arising by 

 proliferation from the germinal epithelium lining the caecum to ova free and fully 

 formed, ready to pass away. 



Each ripe ovum when not deformed by reason of mutual pressure when the 

 tubules are becoming swollen with undischarged ova is of a laterally compressed 

 ovate outline (figs. 35 and 36), the narrow or stalked end having formed the place of 

 attachment to the alveolar wall. Because of this last fact, the narrow neck of this 



