The ultrastructure of the intrapigmented aesthetes which occur in specific areas of the different valves of Callochiton achatinus are described. The possible functions of the aesthete and the pigmented body are discussed in the light of their ultrastructure and the ecological requirements of the animal. The properiostracum in C. achatinus is unusually thick and has a structure very similar in appearance to that of collagen.

With 8 plates and 1 figure in the text) The valve surface morphology and aesthete ultrastructure of Tonicella marmorea is described. The possible functions of these very numerous organs are considered with particular reference to the ecological needs of the animal and their similarity to other invertebrate organs. A periostracum-secreting function is proposed as an alternative to the more commonly postulated light receptor theory. PLATE 111. (a) Dorsal surface of an apical cap showing the layer of periostracum and the fringe of T-shaped bodies. (b) Section through an apical cap showing the ventral and horizontal channels filled with periostracum-like material. All scales in micrometres. P, periostracum; F, fringe of T-shaped bodies. PLATE IV. (a) Ventral region of an apical cap showing cup-shaped depression filled with microvilli produced by cells seen to have a granular cytoplasm containing small, flattened secretory granules. (b) Transverse section in the distal region of a megalaesthete showing the interrelationship between the apical cap, the microvilli and the cytoplasm of the cells which produce them. Interspersed amongst the microvilli are cilia which show the typical 9+2 structure. All scales in micrometres. Ac, apical cap; c, cilium; gc, granular cytoplasm; mv, microvilli; 2, small, flattened secretory granules. PLATE V. (a) Large central cells in the megalaesthete body packed with large, electron-dense secretory droplets. (b)Central cell in the megalaesthete body containing a large, prominent nucleus, mitochondria, RER and golgi, together with secretory droplets with variable contents. Adjacent to this cell is a neurosecretory-like body and a number of microtubules. All scales in micrometres.

The ultrastructure of the aesthetes of Leptochiton asellus is described. Comparisons are made with the ultrastructure of the aesthetes of other chiton species and a number of unusual features are highlighted. In particular the perforated structure of the subsidiary caps and the extensive areas of banded molecular strands are discussed.

Transmission electron microscopy of the spermatozoa and spermatogenesis of 11 species (in three suborders Chitonina, Acanthochitonina, Lepidopleurina) of chiton has shown that each species has a sperm with a unique morphology indicating that spermatozoa can be used as a taxonomic character. Although structure is species-specific, similarities between species within suborders and subfamilies can be recognized. The spermatozoa of species from the suborders Chitonina and Acanthochitonina have a head comprising nuclear material only, the anterior portion of which is in the form of a long thin (approximately 80 nm diameter) filament. In many species the centrioles and mitochondria of the mid-piece are lateral in position, the mitochondria often being sited anteriorly alongside the nucleus. By contrast, Leptochiton asellus , a member of the more ancient suborder Lepidopleurina, has a sperm with a head comprising a nucleus and an acrosome. The mid-piece is also more conven­tional in structure with a ring of five or six spherical mitochondria (sited behind the nucleus) that surround the centrioles. The presence of the acrosome in L. asellus suggests that in the more recent chitons the acro­some has been secondarily lost. It is proposed that loss of the acrosome is correlated to a modification in egg-coat thickness. A preliminary examination of the structure of the eggs of three species has shown that those of L. asellus are surrounded by a very thick chorion (14-30 μm) whereas in Acanthochitona crinitus and Dinoplax gigas there are regions of the chorion that are less than 2 μm thick. The morphological changes that occur during spermatogenesis are very similar in the Chitonina and Acanthochitonina. During spermiogenesis the nucleus elongates to develop a long anterior filament. Chro­matin condensation within the nucleus involves the formation of fibrils that become orientated along its long axis. Closely associated with the elongating nucleus is a manchette. In L . asellus a spherical proacrosomal vesicle appears in the spermatocytes. This vesicle becomes compressed as it matures and simultaneously it migrates to the presumptive anterior end of the spermatid where it invaginates and elongates. Although the pattern of chromatin condensation in the nucleus is similar to that described above, a manchette has not been observed.