io6 



SPONGES 



verted into the smaller spherical ciliated chambers of the third type 

 of canal system. As a consequence the regular tubar skeleton 

 disappears and is replaced by an irregular parenchymal skeleton 

 supporting the chambers and canal system and making up the 

 greater part of the thick body wall, between the cortical and 

 gastral layers of the skeleton. 



One family of Heterocoela deserves special mention, however, as 

 regards its skeleton, namely the Pharetronidae. The anatomical structure 

 of this family is very imperfectly known, since most of its members are 

 fossil, and therefore cannot be studied at all with respect to their canal 

 system, while in many cases even the hard parts are very unsatisfactorily 

 preserved and the finer details impossible to make out. Two living 



FIG. 74. 



Types of tubar skeleton in Sycons. a, articulate type ; b, inarticulate type. (After 

 Haeckel.) 



species are known Lelapia australis, Gray, from the coast of Victoria ; 

 and the remarkable Petrostroma schulzei, Dod., from Japan. From a 

 comparison of the living and extinct forms, the Pharetronidae would 

 appear to be Heterocoela, with a leuconoid type of canal system and 

 with a skeleton of more or less pronounced fibrous structure. The fibres 

 in typical cases are composed wholly or in part of interlocking spicules 

 of a peculiar type, in shape like a tuning-fork (Fig. 72, o). The 

 spicules in question are simply entangled to produce the fibres, and are 

 not held together by any special cementing substance. In Lelapia and 

 Petrostroma the fibres are made up entirely of tuning-forks, but in many 

 fossil forms, as Sestrostomella, they contain an axis or core of much larger 

 and stouter triradiates, and other spicules may enter into their composi- 

 tion. In Lelapia and the fossil forms the fibres ramify through the 

 whole parenchyma, starting from the gastral skeleton and taking an 

 irregular course towards the cortex, so as to produce an anastomosing net- 



