284 



serve to illustrate, however, the degree to which variations in the shell of Teredo are 

 immediately dependent on conditions of the environment. 



The non-genetic nature of such variations is further indicated by the fact noted 

 above (p. 272) that, under certain conditions, rather marked differences may occur 

 between shells from near the upper and lower ends of the same pile. 



There is no reason to believe that variation in Teredo navalis is a phenomenon 

 limited to San Francisco Bay. Rather is it a biological factor to be reckoned with 

 wherever the distribution of this species extends. Such being the case, we would 

 urge extreme caution in the recognition of new species in this group until their claims 

 be established on the most incontrovertible evidence. This is especially desirable in 

 view of the facility with which Teredo navalis invades estuarine harbors and is there- 

 fore subject to variation resulting from the amplitude of environmental conditions. 



Breeding H.\bits of Teredo Navalis 



Teredo navalis is an incubatory species, the young organisms being retained in 

 the gills of the female until a rather advanced stage of development is reached. Fer- 

 tilization of the eggs occurs through the agency of spermatazoa extruded into the 

 water by a male animal, and taken by chance into the mantle cavity of the female 

 with the respiratory current. The fertilized eggs then undergo development in a 

 specialized brood pouch formed in the gills by the fusion of adjacent filaments. During 

 the breeding season the female animals are found normally with the brood pouch 

 distended and packed with minute larvae in various stages of development. The 

 ovaries are at the same time enlarged and filled with unfertilized ova, so that it appears 

 that breeding is continued over a period of weeks, the young escaping from the brood 

 pouch from time to time as they become sufficiently mature. 



The number of larvae which one female may produce in a season has been vari- 

 ously estimated at from 500,000 to considerably more than 1,000,000. The old Dutch 

 naturalist Sellius (1733) estimated the number of ova contained in the body of Teredo 

 navalis to be 1,874,000. A specimen of medium size sectioned by us was estimated, 

 by counting the ova appearing in section over a given area, to contain 750,000 ova. 

 Another specimen was similarly estimated to contain not less than 1,000,000. These 

 estimates being based on the number of eggs contained in the ovaries at one time, the 

 total number produced by the larger females during a breeding season extending over 

 several weeks or possibly months must be greatly in excess of these figures. The marked 

 development of the ovaries during the reproductive period is illustrated in the photo- 

 micrograph, figure 115. In this single cross section somewhat more than 2000 eggs are 

 in evidence. 



The immature eggs in the lobules of the ovaries, as seen in this figure, are irregu- 

 larly pear shaped; but on ripening and prior to fertilization they become spherical. 

 The development of the egg within the brood pouch after fertilization has been de- 

 scribed by Quartrefages (1849b) and in somewhat greater detail by Hatscheck (1881). 

 Cleavage is unequal; the three germ layers are very early differentiated; no cavity is 

 formed in gastrulation. The smaller ectodermal cells grow around the larger blasto- 

 meres which represent the future entoderm and mesoderm, forming thus a ball of 

 cells with the larger inside. The mouth and later the anus are developed by invagina- 

 tion of the outer layer of ectoderm, and finally the inner entodermal and mesodermal 

 cells become differentiated to form respectively the intestine and the muscular system, 

 and also certain other derivatives. In the meantime the shape of the embryo, which 

 at first was nearly round, has become altered. The preoral region is somewhat broad- 

 ened and flattened, while the post-oral portion of the body tapers slightly. The whole 



