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 (Unto, Anodonta, etc.) 

 and the little freshwater Bivalves Sphczrium 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 Area 

 vivipara, Philobrya, the Oyster, which keeps them in 

 the mantle, and the Ship-worm (Teredo) 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, 



