THE SHELL-BEARING SEA SLUGS - SUB-ORDER TECTIBRANCHIATA.
This division embraces families of the Order Opisthobran chiata, in which the shell is seen to disappear by gradual changes, becoming thinner and more enveloped in folds of the mantle and foot as it diminishes in size. The right gill is usually present, but concealed by the mantle fold. The group is not well repre sented in American waters.