The small squid Alloteuthis, and the larger Loligo have elongate mantles chiefly made up of two distinct types of circular muscle fibres, partitioned by thin sheets of radial muscle fibres (Bone, Pulsford & Chubb, 1981).
Nerve terminals on both types of circular fibre and upon radial fibres contain 50 nm electron-lucent vesicles; preliminary pharmacological investigations (Bone & Howarth, 1980) indicate that L-glutamate is the excitatory transmitter at the terminals upon the circular fibres. In other cephalopods, such as Sepia, and the squids Lepidoteuthis and Taningia (Clarke & Maul, 1962; Clarke, 1967) there are in addition to the circular and radial fibres, layers of longitudinal muscle fibres in the mantle. Such longitudinal fibre layers are lacking over the greater part of the mantle in Alloteuthis and Loligo. In this note, we show that they are present only around the anterior margin of the mantle in these two squid, and that both these anterior longitudinal fibres and the radial fibres of the mantle are insensitive to L-glutamate, but contract in response to acetylcholine, as do the retractor muscles of the head and arms, and the siphon muscles.