The embryo is provided with ten hooks, and appears to select Lamellibranchs (Mactra) for its intermediate host.
The best known and most frequent forms are the asari (Tapes philippinarum), the hamaguri (Meretrix lusoria), the baka (Mactra sulcataria), the aka-gai (Scapharca inflata), the kaki (oyster), the awabi (Haliotis japonica), the sazae (Turbo cornutus), the hora-gai (Trilonium tritonius), &c. Among the cephalopods several are of great value as articles of food, e.g.
A pyloric caecum connected with the stomach is commonly found, containing a tough flexible cylinder of transparent cartilaginous appearance, called the " crystalline style " (Mactra).
In some Lamellibranchs (Pecten, Spondylus, Pholas, Mactra, Tellina, Pectunculus, Galeomma, &c.), although cephalic eyes are generally absent, special eyes are developed on the free margin of the mantle-skirt, apparently by the modification of tentacles commonly found there.
Such eyes on the mantle-edge are found in Pecten, Spondylus, Lima, Pinna, Pectunculus, Modiola, Cardium, Tellina, Mactra, Venus, Solen, Pholas and Galeomma.