EEPORT ON THE KERATOSA. 7 



On tlie other hand, in Luffaria the horny laminae and the central differentiation of 

 its fibres resemble each other optically in a far greater degree ; higher powers of the 

 microscope are necessary in order to show this internal differentiation ; when dried, the 

 fibres stUl possess their core, and only after treatment with caustic alkali or ammonia do 

 the fibres become hollow. Certain differences are also to be found in the structure of the 

 pith-substance. While in Aplysina or Verongia the core is represented by a fine and 

 irregularly twined network,^ in Luffaria the structure of the core of its fibres appears 

 similar to that of its horny envelope, being however represented not by continuous 

 laminae, but by minute horny splints, still disposed parallel to one another, as well as 

 with regard to the surrounding lamina, just as Schulze has figured ^ the central canal 

 of the fibres of Spongelia pallescens. But though the core of the skeletal fibres of Luff- 

 ana appears to be structurally quite equivalent to that of Euspongia, and differs con- 

 siderably from that of typical Aplysinidae, it would still be premature to assume that 

 Luffaria is but a specifically modified Spongid, and not a link connecting the Aply- 

 sinidse with the Spongid^, in consequence of the identical manner in which both kinds 

 of fibres, the heterogeneous as well as homogeneous, develop. 



It has indeed been stated — I allude to Dr. v. Lendenfeld's ^ observations — that the 

 development of the heterogeneous fibres only slightly resembles that of the homogeneous 

 ones ; that while, according to Schulze,^ the skeletal fibres of a Cacospongia or EusjJongia 

 grow by reason of the activity of spongohlasts exclusively, the growth of a hetero- 

 geneous fibre is dependent on the function both of spongoblasts and spongohlasts. 

 Like F. E. Schulze, Dr. v. Lendenfeld distinguishes two kinds of spongoblasts — those of 

 elongated, and those of polygonally massive, form ; the first are to be found along the 

 developing fibre, the second on its summit. He thinks, however, that the function of the 

 last-named is not to secrete the pith-substance of the central canal, but to sink down 

 into the interior of the developing fibre, in order to transform into pith-substance the 

 original horny mass, secreted by the elongated spongoblasts (Ich nehme an, dass die 

 ZeUen in den Kuppeln, gleich den Osteoklasten der Wirbelthiere, die harte Einde der 

 Skelettheile auflosen und in Marksubstanz verwandeln). This statement Dr. v. 

 Lendenfeld accompanies by an illustration, and recalls on this occasion the statements 

 of Flemming as to the structure of the skeletal fibres of lanthella, which he (Dr. 

 Lendenfeld) supposes to be very nearly allied to his Dendrilla, and in whose fibres the 

 presence of true cells has been proved. Of course there can be no doubt that the 

 skeletal fibres of lanthella are charged with true cells. The statements of Flemming have 

 been corroborated by Carter, and, for my own part, I can only confirm their observations 

 Yet these cells have been found not in the pith-substance, but between the surrounding 



' Comp. F.E.,Scliulze's statements on tliis point in Zeitschr. f. wiss. Zool, Bd. xxx. p. 401, and my drawing, PI. X. fig. 3. 

 " Zeitschr. f. ^mss. Zool., vol. xxxii., pi. vi. iig. 6 ; comp. my drawing, PI. IX. fig. 6. 

 3 Zeitschr. f. wiss. Zool., vol. xxxviii. p. 291. * Ibid., vol. xxxii. p 635. 



