The porifera and coelentera . ber acquires afirm and rigid skeleton. With the evolution of a leuconoid type of canal system thepronounced radial structure seen in the Sycons becomes lost, andthe elongated radial tubes become very much shortened and con- io6 SPONGES verted into the smaller spherical ciliated chambers of the third typeof canal system. As a consequence the regular tubar skeletondisappears and is replaced by an irregular jXirenchymal skeletonsupporting the chambers and canal system and making up thegreater part of the thick body wall, between the cortical andgastral layers of the
The porifera and coelentera . ber acquires afirm and rigid skeleton. With the evolution of a leuconoid type of canal system thepronounced radial structure seen in the Sycons becomes lost, andthe elongated radial tubes become very much shortened and con- io6 SPONGES verted into the smaller spherical ciliated chambers of the third typeof canal system. As a consequence the regular tubar skeletondisappears and is replaced by an irregular jXirenchymal skeletonsupporting the chambers and canal system and making up thegreater part of the thick body wall, between the cortical andgastral layers of the skeleton. One family of Heterocoela deserves special mention, however, asregards its skeleton, namely the Pharetronidae. The anatomical structureof this family is very imperfectly known, since most of its members arefossil, and therefore cannot be studied at all with respect to their canalsystem, while in many cases even the hard parts are very unsatisfactorilypreserved and the finer details impossible to make out. Two living. Types ;of tub;ir skeleton in ) Via. , articulate type; /, inarticulate type. (After species are known—Lelapia audralis, Gray, from the coast of Victoria ;and the remarkable Petrostroma schulzei, Dod., from Japan. From acomparison of the living and extinct forms, the Pharetronidae wouldappear to be Heterocoela, with a leuconoid type of canal system andwith a skeleton of more or less pronounced fibrous structure. The fibresin typical cases are composed wholly or in part of interlocking spiculesof a peculiar type, in shape like a tuning-fork (Fig. 72, o). Thespicules in question are simply entangled to produce the fibres, and arenot held together by any special cementing substimce. In Lelapia the fibres are niade up entirely of tuiung-forks, but in manyfossil forms, as Ststrostomclla, they contain an axis or core of much largerand stouter triradiates, and other spicules may enter into their composi-tion. In Lelapia and th
Size: 1970px × 1268px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1900, booksubje, booksubjectctenophora
