SPONGES 23 



In flabellate forms the oscula are Usually, if not always, found 

 on one side of the sponge, the inhalant orifices on the other side. 

 Flabellate sponges have a great tendency to become folded until 

 the edges come into contact and undergo concrescence. This can 

 be well seen in such a form as Phakellia ventilabrum, where some 

 specimens are simply fan-shaped, and others are folded into the 

 form of a funnel or cup, in which the surface which bears the oscula 

 is internal (Fig. 35, ). In this way a large series of sponge forms 

 arises which, according to the relative dimensions of different 

 regions, may be funnel-shaped, cup-shaped, or tubular. In the 

 interior are found the true oscula, and on the exterior the inhalant 

 apertures. The sponge colony in these cases exactly resembles the 

 primitive vasiform sponge individual, or some of its numerous 

 modifications, and at first sight the terminal aperture might be 

 taken for a true osculum, the central space for the gastral cavity, 

 and the exhalant vents in the interior for the excurrent openings 

 of the canal system. Hence the cavity in these secondarily cup- 

 shaped or tubular forms has been termed a pseudogaster, and the 

 terminal aperture a pseudosculum. In many cases, however, it is 

 impossible to determine either by simple inspection or by dissection 

 whether a cup-shaped or tubular sponge represents a single in- 

 dividual with a true osculum, or a colony with a pseudosculum. 

 Similarly, a flabellate sponge may represent a colony composed of 

 numerous individuals, or it may be, as we have seen in the case of 

 Euryplegma, a single individual, modified by its peculiar mode of 

 growth. A knowledge of the development can alone decide which 

 view is the correct one in any given instance. 



Another modification of the flabellate type is seen in Phakdlia 

 tenax (Fig. 36) in which the fan has become fenestrated, resulting in 

 a Gorgonia-like form. 



Many deep-sea sponges, especially those of the order Monax- 

 onida, are to be regarded as having migrated downwards from the 

 shore -line in comparatively recent times, and in such forms the 

 influence of life in still water is seen in a great regularity of growth, 

 resulting in the development of a secondary symmetry. A good 

 instance of this is furnished by the remarkable form Esperiopsis 

 Challengeri (Fig. 37). Both the genus and the family (Desmacido- 

 nidae, R. and D. = Poeciloscleridae, Tops, pars) to which this sponge 

 belongs comprise some of the commonest and most characteristic 

 sponges of the littoral fauna, and its nearest allies exhibit the 

 variable and often irregular form which in sponges is associated 

 with life in shallow water. Like its allies, the species under con- 

 sideration is a colony in which the individuals are indistinguish- 

 able, but a more tranquil and uniform environment has favoured 

 a regular and symmetrical growth which is clearly not of a primi- 

 tive type. 



