Shell attachment and associated musculature in the Notaspidea and Anaspidea (Gastropoda: Opisthobranchia)

Abstract

SummaryAn account is given of the muscles of the head‐foot of the notaspidean (Pleurobran‐chomorpha) Berthella plumula (Montagu) and of the anaspidean (Aplysiomorpha) Aplysia punctata Cuvier, with special reference to those concerned in anchoring the shell. This is an internal dorsal plate in both animals.It is shown that parallel evolution has occurred so that in both orders the reduced shell is anchored by a ring of dorsoventral muscles, broken on the right where the mantle complex lies. In addition, some intrinsic longitudinal muscles of the dorsal body wall of Aplysia fasten to the shell margin. In both Aplysia and Berthella the columellar muscle takes the form of a pair of lateral tracts, and in the latter, forms the buccal mass retractor. This retractor muscle, together with some fibres of the left tract, is the only part of the columellar muscle of Berthella to originate on the shell. No such attachment occurs in Aplysia.By reference to this musculature and the gross anatomy, the changes in bodily organization necessary to derive the Notaspidea and Anaspidea from primitive stocks of infaunal cephalaspideans (Bullomorpha), are deduced. The primitive anaspid, Akera, in which the head‐foot has narrowed and partially assumed slug‐like form, provides an annectant example. Upon (1) further reduction in the shell, (2) loss of the posterior mantle lobe and proximal columellar muscle simultaneous with further posterior elongation of the mesopodium and (3) dorsoventral growth of the head‐foot posteriorly, the bodily form typical of the aplysiids was produced. This last change in proportion raised the shell well above the horizontally‐aligned columellar tracts. Slug‐like proportions have also been attained in the Notaspidea by posterior elongation of the mesopodium, but unlike anaspids, there was a simultaneous reduction of the dorsal body wall, so that the anterior mantle border came partially to overlie the rhinophores.It is concluded that the changes in gross bodily form which converted a cephalaspid stock into anaspids facilitate a herbivorous existence, crawling on weed. The adaptations necessary to derive the Notaspidea were involved in the change from an infaunal mode of life to an epifaunal one on either mixed bottoms or hard substrata.