906 THE VOYAGE OF H.M.S. CHALLENGER. 



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. Xanthellse 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 con- 

 formation 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 XL VI. PLAGONIDA, Haeckel. 



Plagonida, Haeckel, 1881, Prodromus, p. 423. 



Plagiacanthida (sensu strictiori), Eichard Hertwig, 1879, Organismus d. Eadiol., p. 72. 



Definition. P lectoidea 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 connec- 

 tion 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. 



Two species only of Plagonida have been hitherto known. The first form described is 

 the Plagiacantha arachnoides, discovered thirty years ago (1855) by Claparede on the 

 western shore of Norway. Another species of the same genus, from the Mediterranean, 

 was very accurately described by Eichard Hertwig in 1879 in his Organismus der 



