92 THE LIFE OF THE MOLLUSCA 
abundant instances occur of incubatory retention of 
the young, usually in the spaces between the gill- 
folds. The freshwater Mussels (Unio, Anodonta, etc.) 
and the little freshwater Bivalves Spharium and 
Pisidium are noted examples; in the two last-named 
instances there are special brood-pouches for the 
reception of the newly hatched young. In the fresh- 
water Zebra Mussel (Dreissensia), on the other hand, 
the young are launched out at once into the water. 
Among marine forms retaining the brood are Arca 
vivipara, Philobrya, the Oyster, which keeps them in 
the mantle, and the Ship-worm (Tevedo) which 
houses them in the gill chamber, while in one 
Bivalve, Thecalia concamerata, the female forms a 
shelly chamber within the margin of the shell for 
the reception and protection of the young. 
The early stages in the development of the young 
Mollusca are the same as in all other animals. The 
original cell splits up into many cells, until the result- 
ing mass looks like a mulberry. The constituent 
cells, however, are unequal in size, one series, the 
first formed, or macromeres, being larger than the 
others, or micromeres, and almost surrounded by 
them. This disparity in size is less marked in 
primitive forms like the Limpet, but most pro- 
nounced in the Cuttlefish, in which the macromeres 
are most highly charged with food-yolk. In the 
Gastropod ovum it is interesting to note that a 
curious obliquity in the cleavage becomes evident, 
