PORIFERA. III. 



disappear does not invalidate the value of this character. - Dendy has also in his subfamily Phloeo- 

 dictyinae the genus A mphiastrella with the species birotulifera Cart.; this species has birotulse for micro- 

 scleres and otherwise a structure almost the same as a Histoderma - - it shows especially a striking 

 resemblance, both in outer shape and in structure, to Histodermella coriacea ; this species certainly 

 belongs to lotrochota, but is a species in which only the dermal spicules are present, and if we form 

 a special genus for it, this genus must be placed near to lotrochota. 



After the manner in which I understand the classification of the above-mentioned forms, we 

 will thus in the various families find the phenomenon, that certain forms have a specially developed 

 dermal skeleton, as a rule together with a reduction of the inner skeleton and occurrence of longer 

 or shorter papillae, by which these forms, in spite of their belonging to different families, get a certain 

 mutual resemblance. It is then an obvious conclusion, that a common factor must be of influence, 

 but it seems not possible in our present state of knowledge to point out such a factor. It is easy 

 enough to understand that all these sponges get provided with papillae, since the hard dermal layer, 

 furnished with close-lying spicules is not adapted to be pierced by pores and oscula and there must 

 therefore be special contrivances for these, viz. the fistulse or appendages, and somewhat similar reasons 

 hold good with regard to the reduction of the inner skeleton. But the cause of the development of 

 the hard dermal layer is therefore not explained, and the only thing, that can be said, is that it seems 

 to be in some connection with the fact, that these forms show a tendency to get free, without attach- 

 ment, or at all events without attachment by a fast-grown basis. The forms which are attached, either 

 with a broad base as Histoderma physa, or by a peduncle as two of the //^//-species, have no 

 specially strong dermal layer; in the quite free forms as most Phloeodictyon-SQecit^ Oceanapia, Histo- 

 dermella, especially coriacea, and Amphiastrclla, the dermal layer on the contrary gets very strong 

 and solid. 



Grayella Cart. 

 (Yvesia Topsent, 1892) 



The external shape varying from thinner or thicker incrustations through massive or cushion- 

 shaped forms to more or less erect and finally quite erect, club-shaped, stalked and sometimes branched 

 forms. The skeleton consisting of, often rather thick but loose, fibres, formed of smooth spicules; in the 

 incrusting and massive forms the fibres radiate from the base upwards and outivards, in the erect there 

 is formed a central axis from which fibres radiate to the surface. The dermal skeleton consisting of 

 tangential, generally very close-lying, spined spicules, making the dermal membrane very firm. Spongin 

 present or wanting. Spicula: megasclera; the skeletal spicules are smooth, they arc most frequently 

 diacttnal, tornota, strongyla or tylota, but they may be monactinal, styli ; the dermal spicules are spined. 

 monactinal or diactinal. Microsclera arc chelce, arcuatce, to which may be added sigmata; sometimes 

 sigmata are found solely, and microsclera may be quite absent. 



