Spines. 



Under the name of >spines« we undersland in this work only a sorl of liollow 

 projections of the onter snrCace of the zoo-cinm, whereas we may designale all 

 solid projections, issuing either from the outer or the inner surface, in a diU'erenl 

 manner, for instance as spinous processes-, »denticles« or in a similar manner, 

 according to Ihc form and size of the projeclions in ([ucslion. Such solid spine-liUe 

 projections are present on the outer surface for instance in HoloporelUi hastigern^ 

 Busk, Hoi cohimnaris', in the species of Farciminaria (PI. 1, figs. 10 a— 10 c) and 

 most species of the genus Spiralaria (PI. 1, fig. 9 c), and on the inner surface e. g. 

 in Menipea roborata (PI. II, figs. 7d — 7 e), Hincks and Men. lignlala (P]. II, fig. 8 c) 

 Mac Gill. From the position, structure and mode of growth we can distinguish 

 between three different main forms, which we may call marginal spines or folded 

 spines, acropctal spines or annular spines and bilaminale spines. 



1) Marginal spinex or folded spines. While these spines may appear in larger 

 or smaller numbers on the frontal wall of species with a membranous frontal 

 area, in the circuit of which they are placed, they may also appear in numbers 

 of 2 — 10 in forms where the frontal area is lacking, outside the anter of the 

 aperture. All these spines originate, as Harmer^ has already found in Memhrani- 

 porella nitida and Crihrilina annuhtta, as crenulalions or folds of the gymnocyst 

 margin, which surrounds the membranous frontal area (PI. IV, figs. 2 a — 2 c) or 

 the anter of the aperture, and the two lateral halves of the fold grow finally to- 

 gether in a longitudinal suture which is turned towards the zoa'cium, and which 

 can often be seen for a long lime even after the spine, by continued growth at 

 the point, has reached its full length. As the fold is closed it comes to enclose 

 a part of the frontal area, and the growing spine will constantly be finished off 

 by a membrane, which is the condition for its further growth in length, and 

 which only disappears when the point of the spine calcifies. These niai'ginal 

 spines, which are always foriued by a Gymnocyst, present a certain likeness in 

 their mode of formation to, the hollow outgrowths of the rim which a])])ear in 

 various snails, for instance in Pterocera chiragra. 



2) Acropetal spines or annular spines. These spines which are only found in a 

 small number of Brgozoa. begin as a ring-shaped growlh on the circum- 

 ference of a rounded uncalcified part of a surface, and grow in other respects 

 in the same way as the marginal spines by means of a membrane at their free 

 end. To these belong the (as a rule) unpaired spine which is situated at the 

 end of the membranous frontal area in the genus Electrn, the unpaired spine in 



' 8, p. 192; - 8. p. 1!)4; ' 19, p. 292. 



