1. The nervous system of cephalopod molluscs provides many unusual features which puzzle the biologist. Among these features is a system of nerves passing to the vena cava. Alexandrowicz proposed that this is a neurosecretory system. 2. Examination of the fine structure of the system in Eledone cirrosa shows that the nerves contain many types of vesicle, the most numerous being electron-dense vesicles of 80 - 150 nm diameter. The vesicles are concentrated in the nerve terminals which lie adjacent to the basement membrane found on the inner side of the blood vessel wall. The appearance of the nerves is similar to that of neurosecretory neurons found in both invertebrate and vertebrate nervous systems. Examination of the fine structure of the system in Sepia officinalis demonstrates that a similar arrangement is also present in this cephalopod. 3. Extracts of the vena cava of E. cirrosa exhibit potent pharmacological activity. This activity may be due to one or more active substances. When assayed on the isolated systemic heart of E. cirrosa the active substance causes an increase in amplitude and a prolonged increase in frequency of heartbeat. The regions of the blood vessel demonstrating this activity exactly parallel the distribution of the nerve terminals within the vena cava wall. 4. Structures within the nerve terminals may be isolated on a discontinuous sucrose gradient. It is found that the cardio-excitatory activity is associated with the electron-dense vesicles of 80 - 150 nm diameter. 5. Gel-filtration of vena cava extracts on Sephadex columns indicates that at least two active substances are present, one with a molecular weight less than 5,000 and one with a molecular weight greater than 5,000. 6. Various techniques, i.e. fluorescence histochemistry, spectro- photofluorimetry, and bioassay, reveal that the activity present in the extracts cannot be attributed to the presence of 5-hydroxy- tryptamine or catechol amines. 7. The active substance resists heating at an acid or alkaline pH, is unaffected by evaporation to dryness, and is extractable in organic solvents e.g. acetone. Further analysis is required before the chemical nature of the substance can be determined. 8. Release of the active substance could not be demonstrated to occur after electrical stimulation of the nerve trunks, or by changing the ionic environment of the nerve trunks. 9. The above results support Alexandrowicz' proposal that the nerves passing to the vena cava in Eledone citrosa form a neurosecretory system. The possible functions of this system are discussed.

This work examines the structural role of the ammonoid shell in supporting hydrostatic load and the role of shell shape in facilitating swimming. The history of studies of the role of the shell in supporting hydrostatic load is discussed first, and is followed by an analysis of the function of the septal suture. The discussion of swimming consists of a critique of the previous work which emphasized the use of Nautilus as a modem analogue for swimming in ammonoids. This is followed by a discussion of flow tank studies that I conducted to examine the role of shell shape in drag production and power consumption of swimming in ammonoids. It had long been thought that the the shells of chambered cephalopods served as buoyancy compensation devices. However, it was not until the 1960's that the internal pressure of the shell was demonstrated to be less than one atmosphere, and that the shell structure supports hydrostatic load. It is remarkable that the internal pressure of cephalopod shells was not ascertained until the 1960s. As early as 1832 the great anatomist Richard Owen called for an empirical investigation of this subject. Functional and adaptive explanations were in vogue in Owen's time as a consequence of William Paley's argument from design. During the late 19th and early 20th century adaptation and functional explanations were no longer emphasized in the interpretation of morphology; non-adaptive evolutionary ideas in vogue in this time period stressed the role of development as a guiding force. It was not until the mid 20th century that there was renewed interest in cephalopod functional morphology and the role of the shell in resisting implosion was ascertained. / Ph. D.