The family #Plectanida# comprises those NASSELLARIA in which the skeleton is composed of radial spines, arising from a common centre, and of a loose wickerwork, produced by concrescence of the meeting branches of those spines. This rudimentary wattled skeleton is either quite irregular or only slightly regular, but it never assumes the form of a complete lattice-shell, as in the #Cyrtellaria# (the #Spyroidea#, #Botryodea#, and #Cyrtoidea#), nor does it exhibit a ring (as in the #Stephoidea#). The central capsule is partly or wholly protected, and often entirely enclosed by the wattled skeleton. Three species only of Plectanida have been hitherto known. The first described form is _Plectophora arachnoides_, which its discoverer Claparède observed in a living state in 1855 on the western coast of Norway, and considered as a mere variety of his _Plagiacantha arachnoides_. Two other species were afterwards observed in the Mediterranean, _Polyplecta dumetum_, 1856, by Johannes Müller (united by him with _Acanthodesmia_) and _Polyplecta polybrocha_ by myself in 1864. Many new forms are found in the {920}collection of the Challenger, so that we may here distinguish eight genera and twenty-seven species. The family Plectanida may be divided into four different subfamilies, according to the number of the radial spines which compose the skeleton. Those four subfamilies, as also the genera contained in them, correspond exactly to the four subfamilies of Plagonida, from which they have originated: Triplectida with three, Tetraplectida with four, Hexaplectida with six, and Polyplectida with numerous (seven to nine or more) radial spines. These spines are usually united (as in the corresponding ancestral Plagonida) in one common central point, upon which rests the basal pole of the central capsule, with the porochora. More rarely (in the genera _Periplecta_ and _Plectanium_) the spines arise in two opposite groups (each of two or three spines) from the two poles of a common horizontal central rod, which supports the overlying porochora of the central capsule.