272 THE BIOLOGY OF THE SEA-SHORE 



Carnivorous Gasteropods deposit their eggs in stout, 

 leathery capsules which often fuse in various ways so as to 

 give a characteristic appearance to the spawn of particular 

 species. Each capsule contains a varying number of ova, 

 and there is competition among the embryos within the 

 capsule, the stronger eating the weaker, so that only a very 

 small percentage survive. A similar struggle, it will be 

 remembered, takes place within the cocoon of the earth- 

 worm. The egg-capsules of Purpura are among the most 

 familiar objects of a rocky shore. In appearance they are like 

 small cereal grains, somewhat pinkish in colour, and they are 

 laid in groups attached by short stalks to the rock surface. 

 A single Purpura has been observed to produce 245 capsules 

 (Cooke, 1895). Equally well known are the egg cases of the 

 common whelk or " buckie " {Buccinum undatum). The 

 specimens of these so often found lying on the shore at or 

 above high-tide mark are, of course, no longer living. 

 The capsules of Fulgur {Busycon) resemble rolls of coins 

 attached at equal distances to a long string (nearly 3 feet) ; 

 Murex deposits clusters of from 15 to 150 triangular capsules, 

 each with a short stalk, and there are said to be about 20 ova 

 in each capsule. Allied species may have very different 

 forms of spawn. Thus the egg capsules of Nassa reticulata, 

 which are fastened in rows to the leaves of Zostera, are shaped 

 like flattened pouches with a short stalk, while those of 

 JV. incrassata are solitary and shaped like rounded oil- 

 flasks (Cooke, he, cit.). 



The manner of formation of the egg capsules of carni- 

 vorous sea-snails has been described by Cunningham (1899), 

 who saw that the spawn was formed and deposited by the 

 foot. The sole-gland is thus really the nidamental gland, 

 and the ova are probably transferred to its cavity before 

 closure and deposition of the capsule. 



The eggs of Natica are laid in a coiled sheet held together 

 by copious mucus in which numerous sand-grains are 

 entangled, the spawn thus resembling a thick sheet of 

 sand-paper (see Fig. 18). 



Nudibranchs . — The spawning of littoral species of 



