On the Structure and Adaptations of the Tellinacea, Deposit-Feeding Eulamellibranchia

The Tellinacea comprise four families of eulamellibranchs possessing common structural features all associated with the habit of feeding on bottom deposits. The habits and general structure of all available British species of the Tellinacea have been studied; in particular, Tellina tenuis (in greatest detail) and Macoma balthica (Tellinidae); Abra alba and Scrobicularia plana (Semelidae); Donax vittatus (Donacidae); Gari tellinella, G. fervensis, Solecurtus chamasolen and S. scopula (Asaphidae). The most significant structures are the very mobile separate siphons formed by fusion of the inner (muscular) lobes of the mantle edge, while the middle (sensory) lobe and the periostracal groove continue around the posterior margin of the shell bordering a deep siphonal space from the base of which the siphons arise. The siphonal apertures are fringed only by six blunt lobes (pinnate and with intermediate lobes in Donax). They are also very insensitive. In the Tellinidae and Semelidae the siphons are very long and the inhalant siphon gropes widely over the surface, actively drawing in bottom deposits. The siphons are shorter and wider in the Donacidae and Asaphidae, but the inhalant siphon opens widely for the passive intake of much material lying on or just above the bottom. The siphonal retractors are highly developed, while the siphons possess a characteristic arrangement of concentric layers of circular and longitudinal muscles with radiating fibres dividing the major layers of longitudinal muscles into a series of bundles. Six nerves, evenly spaced, run down each siphon, always in the same relative position, terminating in the lobes bounding the aperture. In association with the mode of life, the foot is very large and active. It is very much compressed laterally in the Tellinidae and Semelidae, more rotund and pointed anteriorly in Donax, where there is an additional elevator pedis muscle possibly correlated with the habit of burrowing in firm sand, and rounded in Solecurtus (and the Novaculininae), where alone the pedal gape is reduced. Ventral attachment of the siphons in all but the last is reduced to the minimum and takes the characteristic form of the cruciform muscle. This muscle consists of crossed bundles of fibres providing, it is here suggested, an ideal tie for the siphons. The anterior attachments will take the strain when the siphons are extended and the posterior ones when they are withdrawn. There is a sense organ near each of the two posterior attachments consisting of an intramuscular slit opening into a ciliated pit with an associated ganglion and usually communicating with the exterior. Theories concerning the function of the sense organ are discussed and the new view expressed that it is a proprioceptor giving indication of the degree to which the siphons are extended or withdrawn. Need for such information may reside in the presence both of large siphons and of a large foot in the Tellinacea; escape from danger involving first withdrawal of the siphons and then extension of the foot, i.e. unlike the Solenidae or deep burrowers such as Mya in which, respectively, only the foot and the siphons react to danger. In Solecurtus the inner mantle lobes are extensively fused both anteriorly and posteriorly. The cruciform muscle is displaced anteriorly and the pedal gape reduced (hence the superficial resemblance to the Solenidae) and the mantle cavity extended posterior to the shell when the animal is expanded. Broad marginal areas covered with periostracum protect the foot and siphons when these are withdrawn. Conditions in the subfamily Novaculininae are discussed and evidence produced for its inclusion in the Tellinacea in association with the family Asaphidae. The relationships are discussed between Tagelus (with cruciform muscle and no ventral mantle fusion), Solecurtus (with cruciform muscle and extensive ventral mantle fusion) and the Novaculininae (with no cruciform muscle but great anterior ventral mantle fusion). The observations of Atkins (here confirmed) on the ctenidia are discussed. The greatest modifications are in the Tellinidae and Semelidae where the outer demibranch is upturned, consisting of a single surface, and the margin of the inner demibranch has no food groove. These features are associated with the great intake of material. Need to convey this material rapidly forward explains the presence of powerful cirrus-like cilia on the frontal surface of the posterior region of the outer face of the inner demibranchs. These cilia are absent where forward currents exist on the gill axis. In the Donacidae and Asaphidae the gill is less modified, the outer demibranch being reflected although with a supra-axial extension, while there is a food groove on the margin of the inner demibranch; also on the outer demibranch in Tagelus. In general the palps tend to be large when the gills are small, typically in mud dwellers. Pseudofaeces are removed in the normal manner through the inhalant siphons; they are extremely abundant in the Tellinidae and Semelidae in which mantle folds (in all species examined) protect them from being swept forward by the inhalant current, concentrated in the narrow inhalant siphon, while there is an additional ventral channel in Abra and Scrobicularia. In general the gut is longest in species that take much mud into the mantle cavity; faecal pellets of characteristic form are moulded. The initial region of the mid-gut is associated with the style-sac in all but Donax. The stomach is highly characteristic of the group; it can be readily dissected free from surrounding tissues. It is most highly modified in the Tellinidae and the Semelidae. The proximal stomach is elongated vertically so that the style is not bent within it as in suspension-feeding eulamellibranchs. The gastric shield is exceptionally large and strong and carries two massive teeth which bear against the side of the style near its head. The style has been observed to rotate in Tellina tenuis. There is evidence of trituration of large particles presumably between the style and the teeth of the gastric shield. In association with this, a new structure, here termed the postero-dorsal caecum, has been developed to act as a safety valve within which the excess accumulations of large particles are temporarily stored before trituration. The ciliary currents within the stomach are essentially similar to those in other eulamellibranchs, and there is the usual dorsal pouch and food-sorting caecum (this associated with the openings of the ducts into the digestive diverticula). But in general these are less highly developed than in the suspension-feeding eulamellibranchs, but more so in the Donacidae and the Asaphidae than in the two more specialized families. The appearance and probable mode of action of the intact stomach are described and comparisons made with conditions in suspension feeders such as Mya. The only other eulamellibranchs with similarly formed siphons are equally specialized, so that it is necessary to derive the Tellinacea direct from the Filibranchia. The possible course of evolution within the group is discussed. The conditions in Gari are least specialized, those in the Tellinidae and Semelidae, on the one hand, and in Tagelus-Solecurtus-Novaculininae on the other, are the most specialized. Different families, genera and species are adapted for life in all types of bottom material, and are there able to exploit as nutriment the contained organic detritus. The nature of this is briefly discussed, but its importance is best indicated by the variety and number of bottom-living invertebrates that would appear to depend upon it. The Tellinacea are a highly successful group of lamellibranchs which have evolved along characteristic lines with a success revealed by their vast abundance in suitable substrata within both the littoral and sublittoral zones.