860 SPONGES AND ZOOPHYTES 



Lastly there are the least regular megaloscleres, which may be multi- 

 radiate or spherical. 



As is well known, the flesh-spicules are of the most varied forms, 

 and it is a matter of some difficulty so to group them as to render 

 them more easy of comprehension by the student. Messrs. Ridley and 

 Dendy suggest that, provisionally at any rate, we should regard them 

 as (1) simple linear, (2) hooked, (3) stellate. The first may be pointed 

 at either end, and these are often spinous, or they may be long and 

 hair-like, and be or not be arranged in bundles (dragmata) ; a common 

 form is that of a bow, and these, again, are sometimes arranged in 

 bundles, which have been formed within one and the same cell. 

 The hooked forms may be simple sigmiform microscleres, or the 

 shape may be complicated by the inner margin of the shaft or hook 

 thinning out to a fine knife-edge. The most complex forms of this 

 group are the microscleres which the just-quoted authors denominate 

 chelce. They describe these scleres (fig. 655) as having a more or 

 less curved shaft (s), which bears at each end a variable number of 



sharply recurved processes (at, at', It, It'), 

 which they call the ' teeth,' or if broad 

 and expanded the ' palms ; ' these are con- 

 nected with the shaft by a buttress-like 

 xsoo f/l/^b n projection, which is generally so trans- 

 parent as to be with difficulty made out. 

 The shaft itself is frequently drawn out 

 at the side into wing-like processes or 

 fimbriae (/). If the two ends of the 



FIG. 656. Amsochelse of Clado- -, -. z. i * 



rhiea inverse,, showing n, the spicule are equal, we have isochelw ; if un- 

 nucleus of the mother-cell of equal, anisochelcc . The stellate micro- 

 the spicule, from in front, a, sc i er es may be spiral, have a shaft with 

 and from the side, o. x 300. . -,* i* i i i A -^i 



(After Ridley and Dendy.) spmose whorls, or a cylindrical shaft with 

 a toothed whorl at either end. The 



spicules of sponges cannot be considered, like the raphides of plants, 

 as mere deposits of mineral matter in a crystalline state ; for, like 

 all other parts of the organism, they are of cellular origin (fig. 656), 

 and the special cells which produce them are distinguished as silico- 

 blasts ; in this there is first developed a central organic thread 

 around which concentric layers of silica or chalk are laid down. 1 



There is an extremely interesting group of Sponges in which the 

 horny skeleton is entirely replaced by a silicious framework of great 

 firmness and of singular beauty of construction. This framework 

 may be regarded as fundamentally consisting of an arrangement of 

 six-rayed spicules, the extensions of which come to be, as it were, 

 soldered to one another ; and hence the group is distinguished as 

 sexradiate. Of this type the beautiful EuplecteUa of the Manila 

 seas which was for a long time one of the greatest of zoological 

 rarities, but which now, under the name of ' Venus's flower-basket,' 

 is a common ornament of our drawing-rooms is one of the most 

 characteristic examples. 2 Another example is presented by the 



1 For a compendious statement of the characters of sponge-spicules see pp. 82-4 

 of Mr. A. Sedgvvick's Student's Text-booh of Zoology, London, 1898. 



2 The structure and arrangement of the soft parts of ILnplectella aspergillum have 



