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Show 350 MOLLUSCA: ' GASTROPLAX, Blainv.-UMBRELLA, Lam. U t'l the anatomy of this singular genus be more closely investi. n 1 gated, we are compelled to place it a.mong. the T ectl'b ra~c h1' ata, and even near Pleurobranchus. The ammal 1s large ~nd circular, the foot projects considerably beyond t~e mantle, ~nd Its upper sur~ace · t dded with tubercles. The v1scera are m a round, superior, laSn dS cUe ntral part. The mantle is only v1' s1• bl e b y 1. ts s1 1' g h l t y pro· jecting and trenchant edges, along the whole ~f th~ front and of the right side. The lamella ted ~yramidal. branch1re, .hke thos~ of Pleurobranchus, are under this shght margm, and behmd them 1s a tubu· lar anus. Under this same margin and forwards, are two tentacula, longitudinally cleft, as in Pleurobranchus, at ';hose ~nternal base.are the eyes; ~between them is a kind of ?robosc1s, wh1ch may po~s1bly b the organ of generation. There IS a large concave space m the a:terior margin of the foot, the edges of which are susceptible of being drawn up like the mouth of a ~urse, an.d at the bott~rn of which is a tubercle, pierced by an orifice, wh1ch ,per~aps. 1s the mouth, and surmounted by a fringed mambra.ne. 1 he mfenor su.r· face of the foot is smooth, and serves the ammal to crawl on, as m the other Gasteropoda. The shell is stony, flat, irregularly rounded, thickest in the mid· die, with trenchant edges, and marked with slightly concentric strire. It was at first thought to be attached to the foot, but more recent observation has proved that it is on the mantle and in the usual place( I). (1) In the specimen from the British Museum described by M. de Blainville, Bullet. Phil., 1819, p. 178; by the name of G.A.STROPLAX, the shell is, in fact, attached to the under part of the foot, and by what means it is difficult to deter· mine; the mantle, however, is so thin, that it seems as if it must hav~ been P.ro· tected by the shell. M. Reynaud has just brought to France a spec1men which had lost its shell but where, it appears, traces of the membranes which attached it to the mantle ~an be perceived, notwithstanding which, no re~ains ?frouscles are visible. A similar shell is also found in the Mediterranean; 1ts ammal, how· ever, has not yet been observed. GASTEROPODA HETEROPODA. 351 ORDER V. HETEROPODA, Lam.(l) The Heteropoda are distinguished by their foot, which, instead of forming a horizontal disk, is compressed into a vertical muscular lamina, which they use as a fin, and on the edge of which, in several species, is a dilatation forming a hollow cone, that represents the disk of the other·orders. Their branchire, composed of plumiform lobes, are situated on the hind part of the back, directed forwards, and immediately in their rear are the heart and a small liver, with part of the viscera and the internal organs of generation. Their body, a gelatinous and transparent substance lined with a muscular layer, is elongated and usually terminated by a compressed tail. There is a muscular mass belonging to the mouth, and a tongue furnished with little hooks; the resophagus is very long; their stomach thin ; two prominent tubes on the right side of the visceral bundle afford a passage to the freces, semen and ova. They usually swim on their back with the foot upwards(2). They have the faculty of distending their body by filling it with water, in a way not well understood. F orskahl comprised them all in his genus (1) M. de Blainville makes a family of the H:ETEROPOD.A, which he names NEcTOPODA, and unites them in his order of the NucL:EonnANOfliATA with another family that he calls PTEROPODA, and which, of all my Pteropoda, only includes the Limacina. He joins the .IJ.rgonauta with it, on account of some conjecture, of which I am ignorant. (2) This mode of natation induced Peron to think that the natatory blade was on the back, and the heart and branchia: under the belly, and has given rise to many errors as respects the place of these animals. A simple inspection of their nervous system led me to suppose, in my Memoirs on the Mollusca, that they ~ere analogous to the Gasteropoda. A more exact anatomical investigation, made smce then, with that given by M. Poli in his vol. III, fully confirms my supposition. The .fact is, that there is but little difference between the Heteropoda and the Tectlbranchiata, notwithstanding which, M. Laurillard believes their sexes to be separated. |