The pink shrimp Dichelopandalus bonnieri Caullery, 1896, is widely distributed on North Atlantic coasts; it is recorded from the Bay of Biscay, south and west Norway, Scottish and Irish Coasts, South of Iceland and the east coast of North America on muddy substrates in depths ranging from 40 m to 1200 m (Kemp, 1910).

‘During the month of July 1900 the Lord Commissioners of the Admiralty consented, at the request of the Council of the Royal Society, to place H.M.S. “Research” at my disposal for an investigation of the plankton of the Bay of Biscay’ (Fowler, 1904). The area of study was 78 x 20 nautical miles and bounded by the latitudes 46° 43' N and 47° 29' N and longitudes 7° 15' W and 8° 18' W. Fowler (1909) subsequently worked up the halocyprid Ostracoda from tow-net samples taken either with a 20 in (45 cm) diameter net with either 45 or 60 mesh (i.e. pore apertures of about 0.50 or 0.35 mm), or a mesoplankton-net that could be opened and closed. The nets were lowered to depth and left dangling while the ship drifted. This was the first attempt to study the vertical distribution of halocyprid ostracods, and the only study considering this planktonic group in the Bay of Biscay for another 74 years, apart from Colman (1962), who identified the contents of his plankton samples only to Order. Fowler's results were dogged by his taxonomic misconceptions (Skogsberg, 1920). He believed that many of the larger species were later moult stages of smaller ones, e.g. he synonymized C. macrocheira Müller with C. magna Claus. Fortunately he illustrated most of the forms he was dealing with and it has been possible to re-ascribe them with reasonable confidence.

In most fish groups, a large pair of Mauthner neurons provides a distinctive and conspicuous pathway linking the motoneurons of the cord to nerve VIII input. Physiological studies have been almost entirely on the apparatus in teleosts (review by Diamond, 1971), but its morphology is essentially similar in other groups, and it seems likely that wherever found in gnathostomes, its function is the same: to mediate a startle response involving a single rapid tail flip. It is therefore, as Whiting (1957) has emphasized, an important part of the reflex system in those lower chordates in which the myotomal musculature is used in locomotion. Not surprisingly, where locomotion is carried out in a different way (as for example in adult anura), the Mauthner neurons of the larva degenerate and disappear, as they do in a variety of teleosts where a tail flip startle response is inappropriate in the adult.

In most fishes the fibre types of the axial muscles (musculus lateralis) are organized in separate groups. Commonly, there is an outer layer of red fibres of small diameter, medially there is a layer of larger, white fibres. With some modifications this scheme holds for the hagfish (Flood & Mathisen, 1962; Korneliussen & Nicolaysen, 1973), for sharks (Bone, 1966; Hagiwara & Takahashi, 1967; Brotchi, 1969) and for teleosts (e.g. Nishihara, 19673 Rayner & Keenan, 1967; Walker, 1971; Nag & Nursall, 1972; Patterson & Goldspink, 1972). The tail muscles of salamanders are similarly arranged (Sherkov, 1970; Totland, 1976 a, b).

It is well known that intertidal animals exhibit temporal patterning of behaviour in which locomotor activity occurs at particular times of day and state of tide. Naylor & Atkinson (1976) postulate that such behavioural rhythms are partially controlled by endogenous timing processes which determine the approximate periodicity of a rhythm, as expressed in constant conditions in the laboratory, and that these circadian and circatidal rhythms are entrained by environmental variables which in nature synchronize the expression of behavioural rhythmicity at adaptively appropriate times. Several tidal variables, such as temperature, pressure, light and wave action, have been shown to synchronize and entrain endogenous rhythms (Palmer, 1974; Naylor & Atkinson, 1976) but so far there appears to be little evidence of entrainment of tidal rhythmicity in coastal animals by variations in salinity. A tidal rhythm of responsiveness to water of reduced salinity has been demonstrated by Arnold (1970) in the cirral activity of Balanus balanoides (L.), but entrainment by this response in Balanus has not been tested. In any event the tidal rhythm of cirral activity in Balanus balanoides on the shore is not expressed in barnacles kept in constant conditions in the laboratory (Southward & Crisp, 1965) and therefore appears to be largely controlled by exogenous factors. It therefore seemed worthwhile to investigate the effects of salinity changes upon the locomotor rhythm of Carcinus maenas (L.), the control of which by internal factors (Arechiga, Huberman & Naylor, 1974) and some external factors (Naylor & Atkinson, 1972) is well documented.