374 M. K. A. Zittel on Fossil Calcispongice. 
canaliculi penetrate from one or both surfaces into the wall, 
without traversing it. 
Not unfrequently definite canals are entirely absent; and 
this phenomenon is observed both in forms with a tubular 
stomachal cavity, and in those without any oscula and stoma¬ 
chal cavities. In all cases where the canals are wanting the 
skeletal tissue presents a looser constitution, which enables the 
circulation of water to go on without hindrance. 
Exceedingly peculiar phenomena of water-circulation are 
met with in the genera Verticillites and Celyplua. In the 
first-named genus the cylindrical individuals with a tubular 
stomachal cavity consist of hollow rings placed one upon the 
other, and of which the cover of one always serves as the 
base of the next. Only the walls of these rings consist of 
spicular fibres; the interior is hollow. From the cavities of 
the rings fine orificial canals lead into the stomachal cavity ; 
and they are placed in communication with each other by 
similar canals. In Celyphia the stocks are composed of hol¬ 
low globular individuals which possess no common stomachal 
cavity, and communicate with the outer world only by small 
round apertures. 
Skeleton and State of Preservation .—These have been 
already described in detail; so that nothing essential remains 
to be stated in regard to the fibres and their structure. A 
noteworthy peculiarity of the Pharetrones is that isolated 
surface-spicules of characteristic form or remarkable size 
never occur in them. The fibrous skeleton either comes naked 
and unprotected to the surface, or it is enveloped by an ex¬ 
ternal smooth dermal skeleton , which, however, does not con¬ 
sist, as in the Lithistidse, of differently formed surface-spicules, 
but of exactly the same small spicules as the whole of the rest 
of the skeleton. To the naked eye the dermal layer of the 
fossil calcareous sponges appears smooth or concentrically 
wrinkled, and exactly in agreement with the so-called epitheca 
of the corals. It has therefore hitherto always been desig¬ 
nated by the same name. It is rarely, however, that one 
can succeed in demonstrating its spicular structure, as it is 
precisely at the surface that the chemical influences have been 
particularly effective, and have generally led to the complete 
destruction of the skeletal elements. 
Classification .—We must not for the present attempt a 
special classification of the rich and varied family Pharetrones, 
as the skeletal spicules can be observed with sufficient dis¬ 
tinctness only in a few genera. In general we have to con¬ 
tent ourselves with ascertaining the actual existence of spicules; 
and it is only exceptionally that we are able to distinguish 
