798 THE ANIMAL KINGDOM. 



radially to it, and the two sets of cells are in continuity. The cortex 

 increases by the addition to the apex of the fibre of caps of spongin 

 prolonged laterally into lamellae. A certain proportion of medulla, derived 

 probably by the metamorphosis of some of the polygonal cells in the apical 

 cap, is inclosed between successive caps of spongin. The medulla becomes 

 as a rule continuous throughout the system of fibres. A structure closely 

 resembling that of the fresh formed fibre is retained in Aplysilla, Aplysina> 

 and Dendrilla 1 . 



Foreign inorganic bodies, such as grains of sand, spicules of other 

 sponges, &c., commonly occur in the medulla of the fibres, either of the 

 principal alone, or of all without distinction, in the Spongidae and Spon- 

 gelidae. They are probably taken up by the caps of polygonal cells from 

 the mesoglaea. Their amount may be trifling or considerable, and the 

 connecting spongin much reduced and of soft consistence, e. g. in Dysidea, 

 Psammoclema : or it may even disappear altogether as in Psammopemma. 

 Foreign bodies frequently occur scattered through the mesoglaea, taken 

 up from the surface of the sponge, and it has been supposed that a given 

 sponge exercises a selective action on the material imbedded 2 . The 

 peculiar ' skipping rope ' fibres, i. e. filaments with a knob at each end, which 

 occur dispersed in variable quantities in the mesoglaea of certain Ceratine 

 sponges, are supposed to be either parasitic organisms or pathological 

 formations caused by the presence of parasitic algae. They attain some 

 length, e. g. 8 10 mm. (f in. circa] in Hircinia variabilis, and were at one 

 time thought to be characteristic of a family Hircinidae*. 



1 Cells, probably imbedded spongoblasts, are found between the lamellae of the cortex in 

 lanthetta (Darwinellidae} and in the medulla of Dendrilla. According to von Lendenfeld, who 

 has studied the fibres of Dendrilla, these cells, which form layers between the caps of spongin, are 

 destructive in nature, hence spongoclasts. They destroy the caps of spongin and the inner layers 

 of the cortex, and grow centrifugally, retaining their cap-like disposition. They also bore through 

 the cortex at a spot where a branch is forming, and give origin to its medulla. Their powers 

 become exhausted at last, and then they give origin to caps of spongin. Hence though the structure 

 of the fibre resembles that which it has when first formed, it is not absolutely identical with it. 

 Aplysilla violacea and Aplysina resemble Dendrilla in the structure of their fibres. The exact mode 

 of formation of the medulla, how it becomes continuous throughout the fibres, how it increases, if it 

 does increase in other sponges, are points unknown. See on the subject, von Lendenfeld, Z. W. Z. 

 xxxviii. pp. 267, 285, 304; Id. Z. A. viii. p. 469; Polejaeff, Keratosa, op. cit. pp. 4-12. 



2 On the inclusion of foreign bodies, see Schulze's remarks, Z. W. Z. xxxiii. p. 14. In Halme 

 nidus Vesparum (Auleninae) the cortex of the sponge is filled with sand grains, large on the exposed 

 portions, small in the sheltered or internal, cemented into a crust by spongin. Dysidea, Psammo- 

 clema, Psammopemma are described by Marshall, Z. W. Z. xxxv, together with Phoriospongia. The 

 last-named affords an instance of a skeleton of foreign bodies, plus proper siliceous spicules. Some 

 true homy sponges possess spicules ; see von Lendenfeld, Proc. Lin. Soc. New South Wales, ix. p. 493 ; 

 x. p. 490. Hence it is possible that Phoriospongia is a Ceratine in which the spongin is lost, as 

 in Psammopemma. The relationship of the horny to other sponges is a vexed question. See 

 Polejaeff, Keratosa, op. cit. p. 76 et seqq.; Vosmaer, Biol. Centralblatt, vi. p. 187 ; vou Lendenfeld, 

 op. cit. supra, x. p. 483 ; Id. Z. A. vii. p. 201 ; ibid. viii. p. 484. 



3 See Schulze, Z. W. Z. xxxiii. p. 19; Polejaeff, Keratosa, op. cit. pp. 12-16; and for the 

 second view mentioned, von Lendenfeld, Z. A. viii. p. 483. Carter maintained, in 1878, the view 



