366 Mr. Robert Gamer's Malacological Notes. 



the tubular shell, which has also a perforated disk or rose in 

 front, in w-hich is a small central slit for the attenuated foot, 

 and also having a raj of shelly tubules around the margin. 

 These animals, compared with ordinary bivalves, and with 

 tubicolous annelids, are another instance showing the differ- 

 ence between homology and analogy, the latter occurring, as 

 observed already, in very different animals where the circum- 

 stances of life are similar. Like Teredo^ Aspergillum is in 

 every respect a bivalve conchifer, with the mantle, however, 

 closed in front, but having there a thickened muscular disk 

 with tentacular processes for the perforations above mentioned, 

 and also giving exit to the foot, as does the corresponding 

 shelly plate. AVe should suppose that the animal lives buried 

 in fine sand or mud, in which its rayed fringe must secure it, 

 as the expanded foot of some other bivalves does. 



With the exception of Aspergillum and one or two allied 

 genera with fused valves, adductor or janitor muscles are 

 general in bivalves, and also peculiar to them — unless we hold 

 Oken's theory that the operculum of Gastropods is in reality 

 one of the altered valves of the bivalve, in which case the 

 retractor muscles of the former may also include the adduc- 

 tors*. The muscles of attachment of Patella, Fissurella, Hip- 

 ])onyXj NaviceUa, Sigaretus, and Haliotis show the transition 

 from the retractor pedal muscles of the bivalve to the more 

 or less united and contorted muscle of the spiral gastropod. 

 There is sufficient resemblance between an acephalous and a 

 cephalous mollusk to proclaim them of the same division of the 

 animal kingdom ; but the latter, with its more or less marked 

 head, has generally its nervous system more concentrated into 

 a brain or cephalic ganglion, with defined auditory sacs, more 

 developed eyes, and, in some, organs of smell. In Acephala, 

 when mature, no eyes, except the ocelli at the margin of the 

 mantle, exist. The lips and labial palps of bivalves are trans- 

 muted in Gastropods to subulate or flattened tentacles, or some- 

 times into supra- or infraoral disks or processes. Horny jaws 

 of varied form, median or lateral, but the representatives of the 

 beak of the cuttlefish, and a spiniferous tongue or odontophore 

 (Huxley) may exist, the latter almost universally, though 

 it does not appear to have been found in the Pyramidelloe. 

 The hooks or teeth from this tongue do not lose their form 

 when boiled in nitric acid or when calcined, but vitrify with 

 potash, which is perhaps conclusive as to their siliceous nature. 

 The branchiae are so varied in position that they were chosen 



* This theory is strongly advocated by Macdonald, Journ. Linn. See. 

 vol. V. no. 18. "He considers the operculum to be homologous with the 

 right valve (see further on ). 



