PORIFERA. 791 



except in Clione (? Clionidae). Discontinuous gemmation occurs in two 

 forms. Colonial (?). Marine with the exception of the freshwater Spon- 

 gillidae. 



The external outline of the body even when most constant and 

 characteristic, e. g. in some Calcarea, many Hyalospongiae, is not uniform 

 as it is in other Metazoa. Indeed, it is extremely variable in most 

 instances, the variability amounting to Polymorphosis, or change of form 

 without a corresponding physiological division of labour ; see p. 238. The 

 variability in question depends chiefly on two factors : (i) an irregular 

 mode of growth ; and (2) on concrescence or a fusion of parts belonging 

 to the same sponge, the same colony (?), or to sponges of the same species 

 growing hard by one another. Such fusion frequently leads to the en- 

 closure of spaces really external to the sponge-body, which form a false 

 gastric cavity (pseudogaster) opening by a false osculum (pseudosculum 

 s. pseudostome) and false pores (pseudopores). Anatomy, especially that 

 of the gastric and skeletal systems, affords at present the only characters 

 "of classificatory value. 



The primitive Poriferan, as seen in the simple form (Olynthus) of 

 the Asconid Leucosolenia (Ascettd] primordialis, is a vasiform individual 

 with an osculum or aperture at one end and a peduncle of attachment 

 at the other ; with thin body-walls perforated by numerous pores, and 

 a gastric cavity of the same shape as the body, lined throughout by 

 collared and flagellate endoderm cells. Such simple forms are few, and 

 the external shapes assumed by Poriferans are very numerous and may 

 differ at various stages of growth. A radial symmetry may be acquired 

 by the development of conular outgrowths from the body or by the 

 arrangement of the skeleton. But a sponge may be a shapeless or sub- 

 spherical mass, tubular, cup or saucer-shaped, columnar, leaf or fan-like, 

 branched or dendriform. It is very rarely, if ever naturally, free ; but 

 either adherent to some foreign object or rooted in sand or mud by basal 

 processes or by spicules as in many Hyalospongiae. The Calcarean Homo- 

 derma is an exceptional instance of the union of individuals by a creeping 

 tubular stolon. The Clionidae bore into shells or stones 1 . The thickness 

 of the body- wall undergoes great increase, and its consistence is variable, 

 both depending largely on the character of the mesoglaea and the skeleton. 

 And the gastric cavity undergoes at the same time great changes. 



The ecto- and endo-derm are unilaminar. The former consists as 

 a rule of flattened, rarely cubical, polygonal cells, with clear contents 



1 Nassonow states that the larval Clione commences to bore into a shell as soon as it fixes 

 itself, and before any skeletal structures are developed. It throws out slender processes, which per- 

 forate the shell-substance and cut out small semi-ellipsoidal pieces, which are taken up and then cast 

 out by the sponge. The penetration of the shell-substance is effected beyond doubt by chemical 

 means. See Z. W. Z. xxxix. pp. 297-300. 



