It is absent in the most primitive and symmetrical forms, such as Haliotis and Pleurotomaria.
The heart in Patella consists of a single auricle (not two as in Haliotis and Fissurella) and a ventricle; the former receives the blood from the branchial vein, the latter distributes it through a large aorta which soon leads into irregular blood-lacunae.
Limpet, and that of the g ' nerves which pass from the visceral loop of Haliotis to the olfactory patch or osphradium, which lies in immediate relation on the right and on the left side to the right and left gill-plumes (ctenidia) respectively.
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.
This is not an unusual arrangement in Lamellibranchs, and a similar disposition occurs in some Gastropoda (Haliotis).