A surface anisotropy has been shown previously to be induced in thin films of photoreactive coumarin side-chain polymers by polarized UV illumination. Consequently, the resultant cross-linked polymer layers can be used as photoalignment layers for liquid crystal displays. Homogeneous alignment of a nematic liquid crystal in contact with a layer of a model coumarin side chain polymer is obtained with the director parallel or perpendicular to the UV polarization axis depending on the incident fluence. Spectroscopic analysis of the alignment layer now confirms that both photodegradation and cross-linking occur with different dependencies on fluence. Low UV fluences give parallel photoalignment and high cross-linking reactivity. However, the residual, unreacted polymer side chains show negligible anisotropy because of their freedom to move in an isotropic fashion. Hence, parallel liquid crystal alignment is attributed to a steric interaction between the liquid crystal and syndimerized side chains of the cross-linked polymer. A switch of the photoalignment direction accompanies the subsequent development of anisotropy of the intact, unreacted polymer side chains. The side-chain anisotropy and hence perpendicular liquid crystal alignment is ascribed to photodegradation rather than cross-linking.

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Following an examination by transmission and scanning electron microscopy of the latero-frontal tracts on the gills of Nucula sulcata, Ostrea edulis and Chlamys varia , it is suggested that only two types of structure are involved, namely compound eu-latero-frontal cirri and pro-laterofrontal cilia; the terms ‘anomalous latero-frontal’, ‘para-latero-frontal’ and ‘micro-latero-frontal’ should be dropped. Each latero-frontal tract of N. sulcata consists of a row of compound eu-latero-frontal cirri and four rows of pro-latero-frontal cilia. Each cirrus is borne by a single cell and consists of some 20 pairs of cilia arranged in two parallel, alternating rows. Individual cilia leave the shaft of the cirrus at regular intervals on each side but there is no stiffening element present in the region of the bend. Each latero-frontal tract on the plicate, heterorhabdic gill of O. edulis consists of a single row of compound cirri and two alternating rows of pro-latero-frontal cilia. The cirri of the principal filaments are spaced 1.5- 2.0 pm apart and consist of 10 or 11 pairs of cilia. Those on the ordinary filaments forming the crests of the plicae are spaced 2.5 μm apart and consist of but 6 or 7 pairs of cilia. As in N. sulcata , the individual cilia bend to either side of the main axis of each cirrus but, unlike those of N. sulcata , a small stiffening element is present in the immediate region of the bend. It is concluded th at the Ostreidae should not be grouped with the Microciliobranchia. The latero-frontal tracts of the plicate, heterorhabdic gills of C. varia consist of a single row of pro-latero-frontal cilia only. In both O. edulis and C. varia , mucous glands and sensory ciliary tufts occur mainly along the frontal faces of the ordinary filaments forming the crests of the plicae; the glands of O. edulis appear to contain a neutral mueoprotein and those of C. varia an acid mucopolysaccharide. The principal filaments of C. varia are capable of marked changes in form with consequent effects on the nature of the plication of the gill. When few particles are presented to the gill the principal filaments are U-shaped in section and form a pronounced gutter at the base of the grooves between adjacent plicae. It is suggested that collection of particles for possible ingestion is by way of water currents which flow dorsally in the U-shaped principal filaments rather than by any straining effect by the latero-frontal tracts. Correlated with this method of collection the frontal cilia of the principal filaments are arranged in three well defined tracts, all of which beat dorsally. When the concentration of particles presented to the gill is increased, the principal filaments temporarily lose their U-shape and, in extreme cases, become T-shaped in section; the current flow referred to above thus breaks down. Increased amounts of mucus are secreted by the glands associated with the ordinary filaments and mucus, plus entangled particles, are carried ventrally to the free margins of the demibranchs by the frontal tracts of the ordinary filaments. It is suggested that the collection of particles by way of water currents, as proposed for the gill of C. varia , has been developed in those bivalves in which the latero-frontal tracts consist of pro-latero-frontal cilia only, that is the Microciliobranchia excluding the Ostreidae. In those lamellibranch bivalves in which the latero-frontal tracts consist of compound eu-latero-frontal cirri in addition to pro-latero-frontal cilia, the collection of particles is achieved primarily by the straining action of the cirri and the retained material is transported by direct ciliary action rather than in suspension by water currents.

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The epithelium lining the digestive tubules of Cardium edule consists of three cell types, namely mature digestive cells, mature secretory cells and immature flagellated cells. Both the secretory and flagellated cells exhibit a pronounced basiphilia and occur in well-defined crypts. The secretory cells are pyramidal in shape and characterized by the possession of a well-developed granular endoplasmic reticulum and Golgi apparatus. Golgi vesicles derived from the latter migrate to the apical region of the cell where they release their contents into the lumen of the tubules. It is possible that the secretion contains enzymes and although it is likely that such enzymes would function primarily in the lumen of the tubules they may also be the source of the weak proteolytic activity which has been recorded in the gastric fluid of many bivalves. The immature flagellated cells are columnar in shape and possess a poorly developed endoplasmic reticulum and numerous free ribosomes. Although no evidence for this was obtained it is suggested that they may serve to replace either or both of the mature cell types. The digestive cells vary from cuboidal to columnar, possess distinctive Golgi elements with characteristic intracisternal membranous elements, and are capable of ingesting exogenous material from the lumen of the tubule. The process of ingestion was examined following feeding experiments with (a) a mixture of iron oxide and colloidal graphite (Aquadag), (b) whole blood from pigeon and (c) ferritin. Individual particles of graphite were enclosed in phagosomes by a process of phagocytosis, while the proteins haemoglobin and ferritin were ingested by a process of pinocytosis; the membrane enclosing the pinocytic vesicles possesses a characteristic outer granular coat. The contents of both the phagocytic and pinocytic vesicles were transferred to larger bodies considered to be primarily phagosomes in the sub-apical regions of the cell. These possess an interconnecting system of membrane-bound channels which ramifies through the apical cytoplasm. Phagolysosomes deeper in the cytoplasm of the cell were identified by the presence of exogenous material and a positive reaction to tests for acid phosphatase activity. They showed changes in appearance which could be put into a series suggestive of the progressive intracellular digestion of the ingested material.

Adjacent eu-latero-frontal cirri beat alternately and alternate cirri exhibit metachronous activity. Each cirrus consists of 22─26 pairs of cilia arranged in two parallel but alternating rows. Individual cilia leave the shaft of the cirrus at regular (0.6 μ m) intervals on each side to extend across the intercirrus space. Each cilium, where it bends to leave the cirrus shaft, contains a stiffening rod which extends 1.5 μ m distally as a tapering, hemispherical rod, lying between peripheral fibril 1 and the paired central fibrils. Distal to the bend, the peripheral fibrils are singlets and lack arms. It is concluded that the disposition of this region of the cilium to the cirrus shaft remains more or less constant during both the effective and recovery strokes of the cirrus and this disposition is such that interference between adjacent cirri is minimal while permitting effective filtering of the space between alternate cirri. The mesh of the filter is 0.6 μ m by either 2.4 μ m or 4.8 μ m, the latter depending on the relative positions of adjacent cirri. The pro-latero-frontal cilia may serve to transfer material from the eu-latero-frontal cirri to the frontal cilia of the gill filament.