Ill PHYLUM AND CLASS PORIFERA 113 



family Sycettidce is one of several families of the order Heteroccela 

 of the class Galcarea. Among the families of the Heteroccela, 

 that of the Sycettidce is distinguished by the following features, 

 which characterise all its members : — 



" The flagellate chambers are elongated, arranged radially around 

 a central paragastric cavity, their distal ends projecting more or less 

 on the dermal surface, and not covered over by a continuous cortex. 

 The skeleton is radially symmetrical." 



Of the genera into which the Sycettidce are divided, Sycon is 

 characterised as follows : — ■ 



" The flagellate chambers are not intercommunicating ; their 

 distal ends are provided each with a tuft of oxeote spicules." 



The members of one of the other genera of the family — Sycetta— 

 while possessing the general characteristics of the family, differ from 

 those of the genus Sycon in wanting the tufts of oxeote spicules ; 

 those of a third — Sycantha — have the flagellate chambers united 

 in groups ; the chambers of each group intercommunicating by 

 openings in their walls, and each group having a single common 

 opening into the gastric cavity. The members of this genus re- 

 semble Sycon, and differ from Sycetta, in the presence of tufts 

 of oxeote spicules at the distal ends of the flagellate chambers. 



These distinctions between classes, orders, families, and genera 

 are of an entirely arbitrary character. No such divisions exist in 

 nature ; and they are merely established as a convenient way of 

 grouping the sponges and facilitating their classification. But a 

 classification of this kind, if carried out on sound principles, should 

 nevertheless have something corresponding to it in nature, inas- 

 much as the grouping of the various divisions and subdivisions 

 aims at expressing the relationships of their members to one 

 another. The members, for example, of the family Sycettidce are 

 all regarded, on account of the features which they possess in 

 common, as being more nearly related to one another than to the 

 members of the other families, and as- having been derived from a 

 common ancestor which also possessed those features — the diver- 

 gences of structure which we observe in the different genera and 

 species being the result of a gradual process of change. 



Within the limits of the genus Sycon, S. gelatinosum is distin- 

 guished from the rest as a group of individual Sponges all possess- 

 ing certain specific characters which it will be unnecessary to 

 detail here. But the individual Sponges referable to this species 

 frequently differ somewhat widely from one another : there are 

 numerous individual variations. If we compare a number of 

 specimens all possessing the species-characters of Sycon gelatino- 

 sum, we find that they differ in the number of branches, in the 

 shape of the cylinders — some being relatively narrow, some re- 

 latively wide — in the degree of development of the oscular crown 

 of spicules, in the ratio of the thickness of the wall to the width 



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