PORIFERA. II. j-.r 



large, about circular, arched plate, and it forms about a right angle with the shaft. No falx is devel- 

 oped. When the bipocillum is viewed from the front, it conveys the impression of consisting of two 

 cup-shaped plates connected by a shaft, which has given rise to the common description of it. The 

 lower, spoon-shaped end of the bipocillum is most frequently or always finely indented in the edge, 

 which is best seen in the largest specimens. From this typical form of the bipocillum deviations may 

 be found. Thus in species where the bipocilla are otherwise of the typical form, forms may occur 

 with the tooth split to two or three teeth, as for instance in the /. piceus treated below; the splitting 

 is often irregular, and also other deviations in the form may occur. Then the bipocillum may con- 

 stantly be of a different form, as in I. chelifer, where the tooth is always split to three or sometimes 

 two lobes, and the lower end is prolonged to two points. — Both forms of microscleres seem to be 

 rather varying in the same species with regard to their frequency; in some individuals they are found 

 in great numbers, in others they are scarce, and sometimes one form, sometimes the other is the most 

 frequent one. This variation seems to be most marked with regard to the bipocilla, which may some- 

 times be seen only quite singly. 



The lop/iou-species have always, whether they are preserved in spirit or dried, a very dark, 

 sometimes quite black colour, and this feature has sometimes been used as a character of the genus. 

 This colour, however, as has especially been pointed out by Topsent, is not original, but they are 

 in the fresh state of the common colour, yellow to lightly brown. In the air or in spirit they assume, 

 however, the dark colour. Sometimes specimens may keep the original colour more or less markedly. 

 This feature of the colour getting dark after death is also found in Pocillon. 



While Ioplion is distinctly characterized as genus, the same cannot be said to hold good with 

 regard to its species. Ridley and Dendy have determined part of the Challenger-material as Iophou 

 Pattersoni, and in this species united several of Bowerbank's species, of which, however, Hyndmanni 

 and scandens in reality belong to Pocillon. Topsent is more inclined to embrace the contrary opinion 

 that the question is of several species. The fact is that the /op/iou-species, or, at all events, some of 

 them, may evidently be somewhat polymorphous. The microscleres yield no good specific characters, 

 both the chelae and the bipocilla in their typical form being exceedingly uniform in the different 

 species. The styli may in the same species vary very much with regard to spinidation, from about 

 quite smooth needles to highly spined ones. All these features make it a difficult thing to separate 

 the different species. A sure distinction will, however, generally be obtained, when characters are 

 drawn as well from the spiculation, the skeletal structure, and the exterior, as from the structure of 

 the dermal membrane. Thus among the species described below, I. piceus is somewhat polymorphous 

 both with regard to form and spiculation, but nevertheless it is a well characterized species, and the 

 same holds good of /. dubius, which is, however, somewhat less polymorphous. 



i. I. piceus Vosm. 

 PI. VI, Figs. i—2. PI. XVII, Fig. 3 a— b. 



1881. Alebion piceum Vosmaer, Niederl. Arch, fiir Zool. Suppl. Band I, 42, PI. I, fig. 19, PI. Ill, figs. 75 

 —78 et 81—82. 



