The physiological value of the skeleton, with regard to the central capsule, is different in the Plagonida and Plectanida; in the former it supports, in the latter it encloses the capsule like a shell. In the Nassellida, where no skeleton is developed, the central capsule is quite free and naked, enveloped only by the calymma. {906}The calymma or the extracapsular jelly-veil in all #Plectoidea# is voluminous, and encloses not only the central capsule completely, but also the skeleton wholly or partially. Its form is of the greatest value for the development and configuration of the skeleton. Sometimes the calymma is alveolate and foamy, as in _Nassella_ and the common _Thalassicolla_. In several other #Plectoidea# the calymma seems to include numerous small vacuoles, sometimes also pigment-granules. Xanthellæ are commonly scattered in it in great numbers. The pseudopodia, arising in a large bunch from the porochora of the capsule, and running along the branches of the radial spines, seem to be always numerous, richly branched, and with a strong tendency to form anastomoses. The peculiar form of their network is often exactly preserved in the conformation of the skeleton, produced by them. The peculiarities of this network require further accurate observations, as does the whole organisation of the #Plectoidea#. _Synopsis of the Families of_ #Plectoidea#. I. Skeleton (originally tripodal) composed of radial spines united in a common centre and supporting the central capsule, without wicker-work, 1. PLAGONIDA. II. Skeleton (originally tripodal) wattled, with irregular wicker-work, composed of the united branches of radial spines and enveloping the central capsule, 2. PLECTANIDA. Family XLVI. #PLAGONIDA#, Haeckel. _Plagonida_, Haeckel, 1881, Prodromus, p. 423. _Plagiacanthida_ (_sensu strictiori_), Richard Hertwig, 1879, Organismus d. Radiol., p. 72. _Definition._--#Plectoidea# with a spiny skeleton, composed of radial spines, which arise from a common central point or central rod, and support the free central capsule. The family #Plagonida# comprises those NASSELLARIA in which the skeleton is only composed of united radial spines, arising from a common centre, without any connection of meeting branches of the spines; the rudimentary skeleton exhibits therefore neither a loose wickerwork (as in the closely allied Plectanida), nor a ring (as in the #Stephoidea#), nor a complete lattice-shell (as in the #Cyrtellaria#, the #Spyroidea#, #Botryodea#, and #Cyrtoidea#). The central capsule, which possesses all the characters of the MONOPYLEA, is therefore free, not enclosed, and only on one side supported or partly protected by the radial spines or their branches.