[25] INSTRUCTIONS FOR COLLECTING MOLLUSKS BALL. 



wbich haunt the canals of CUona and other "bread sponges" are prob- 

 ably commensals rather than trne parasites, and find shelter in these 

 passages while their food is conveniently brought to them by the ciliary 

 currents kept up by the sponge. 



Small mollusks may often be found in the stomachs of starfish as well 

 as other marine animals. In the Arctic regions the walrus, after using 

 his great tusks to rake Astarte and Natica out of their muddy bed, is 

 in the habit of swallowing them whole. The shells, frequently little 

 injured by their journey through the animal, may be found in large 

 quantities on the rocky beaches where the walrus "haul up" on the 

 shore. The roots of kelp and the branches of sea fans and other corals 

 are the peculiar habitat of a few interesting and often very rare forms. 



EGGS AND EGG CASES. 



The eggs of bivalve mollusks are usually protected within the valves 

 of the mother, and when the sexes are separate, as in a majority of bi- 

 valves, this habit results in the modification of the form of the female 

 shell. In some species the larval mollusks cling to the gills of the par- 

 ent, in others there are special pouches modified for their use, and these 

 differences occur in most closely related sx)ecies of one genus. In other 

 groups, as in the cases of Perna, Cardium, Poromya, and Cuspidaria, the 

 gills may be so attached to one another and to the mantle, or the 

 siphonal septum may be so produced forward as to form a special 

 chamber serving as a marsupium for the protection of the young. In 

 gastropods the whelks deposit their eggs in horny or leathery ovicap- 

 sules generally anchored to some stationary body. The prickly coils of 

 the Qgg cases of Fulgur have been already referred to. Buecinum de- 

 posits its capsules in a heaj) like grains of corn but so arranged as to 

 admit the sea water to every part. They are commonly placed on or in 

 some dead bivalve. The ovicapsules of Purpura stand on end, like lit- 

 tle vases in groups upon the rock, usually under the bladder weed. 

 Ghrysodotnus heaps its capsules in a cylindrical tower sometimes six 

 inches high. Voluta and Btronibella have a few large eggs in hemi- 

 spherical or lenticular capsules an inch across. One species has a large, 

 floating, spherical capsule. There are many gastropods whose eggs are 

 not protected in this way. The nudibranchs lay theirs in a jelly-like 

 mass, string, or coiled ribbon among the marine vegetation. The 

 Calyptraeidw protect their smaU yellow ova in a jelly-like mass be- 

 tween the muzzle and the front of the foot under the j)arent shell. At 

 such times a strip or fringe of thin tissue, which runs along each side 

 of the neck, becomes greatly enlarged and is wrapped about the egg 

 mass as a mother would wrap her infant in a blanket. Of the eggs of 

 a great majority of marine mollusks nothing is known, and here there is 

 a most interesting field for study and observation. 



