"TERRA NOVA" EXPEDITION. 



Orynnx. The sexes, as in other Anatinacea, are uiiiteil in the same 

 individual. The limits of the gonad of either sex are not very sharply defined, 

 but the greater part of the ovary is superficial, covering the upper and anterior parts 

 of the visceral mass ; the testis lies deeper, and is mainly situated in the ventral 

 region of the visceral mass, packed amongst and around the coils of the intestine. 

 The genital ducts communicate with the exterior, as mentioned in the description of 

 the renal organs, through a uriuogeuital canal, and in this respect differ from those 

 of Aiuitiua xuln-oxtriitd described by Pelseneer (I6A, p. 72). 



The eggs, while yet in the ovary, are enclosed, as in other Anatinacea, in a thick 

 capsule. In this particular individual many of them had been laid, and were found 

 in masses in the supra-branchial chambers in the neighbourhood of the mouth of the 

 urinogeuital canal. None, so far as observed, had penetrated between the laminae of 

 the inner demibrauch. Among the Auatiuacea are some of the comparatively few 

 marine Lamellibranchs in which the eggs are lodged in the gills during a longer or 

 shorter period of incubation. The gills themselves in this group are, however, not 

 really well adapted to contain any great number of eggs, for their tissues are bulky, 

 their pleating is extremely close, and the interlamiuar space is much restricted. On 

 the other hand, in the present species of Anatina (and in Myochama and Cochlodesmd), 

 owing to the peculiar extension of the gill-axis just described and the fusion of the 

 gill-margins to the body above and below it, an enclosed supra-branchial chamber 

 (Fig. 25, S./M'.) is formed, roomy enough to provide accommodation for a very con- 

 siderable mass of eggs. 



Whether this cavity is used in Myochama and Cochlodesma for the retention of 

 eggs I do not know, but the presence of masses of eggs within it in Ainitinn I'Hi^t'n-ii 

 leads one to suppose that it is so used in that species, and suggests further that the 

 modifications observed in the mode of attachment of the gills may be for the purpose 

 of fitting them to serve as an efficient brood-pouch. 



In concluding this Report, there are a few points that may be recapitulated, as 

 having an interest beyond that attaching merely to the genera in which they 

 respectively occur. 



It will have been noticed that amongst the Arcacea described, three stand apart 

 owing to their close similarity; these are the three mouomyariau genera Lissarea, 

 Adacnarca, and I'liilnln-i/ii. The similarity is not only in the broader features of their 

 anatomy, but also in various lesser and apparently trivial , peculiarities ; such, for 

 instance, as the form and relations to the mantle of the outer palp, the presence of a 

 pair of glandular caeca between the posterior lip and the body, a ridge of modified 

 epithelium between the gill-axis and the body, and in Ailm-narca and Plululrya the 

 details of the relationship of the mantle-cavity to the anterior parts of the gills. 



