THE NATURE OF SPONGES. 
173 
thing is known concerning the way in which physical and 
chemical forces are directed and co-ordinated in living organ- 
isms, and that when we speak of electric or magnetic polarity 
we do not really know that ultimate particles of matter have 
opposite (polar) qualities developed at opposite sides or ends. 
A complete organism may be regarded as composed of two 
parts — the vegetative and the reproductive. These are found 
united in all the higher animals — divided in many of the 
lower. “ Neither alone can represent,” says Professor Clark, 
“the individual unit;” so he employs the term cephalism to 
indicate “ the potentiality of these subdivisions to live apart.” 
One he calls a vegetative, and the other a sexual cephaloid .” 
Strong objections might be taken to this use of the term 
“ cephaloid,” as there is no actual kephale or head in the 
case, and etymology furnishes a hindrance rather than a help to 
understand the word. The present purpose, however, is not 
criticism, but exposition ; and the reader will now be prepared 
to understand what is meant when Professor Clark terms cer- 
tain forms he meets with in spongillse cephalids , and why he 
calls sponges polycephalic animals. The American spongilla, 
which he names arachnoidea , differs from the English form. It 
resembles an irregular spider-web. “ It lives in fresh-water 
streams and ponds, usually at the bottom of the stems of 
water plants, or wherever there is considerable shade ; appa- 
rently avoiding the light, as we seldom if ever found it in open 
water. In size it varies from a few inches to half a line in dia- 
meter ; of no definite shape ; and has a uniform fuscous or 
yellowish colour, and is wrapped about by a filmy transparent 
colourless envelope (investing membrane of Carter). The brown 
colour is inherent to the interior mass, in which groups of 
monads are imbedded. The investing membrane is also slightly 
tinged with amber colour by the large and small spicules which 
are embedded in it.” It is only in minute specimens, free from 
extraneous matter, that the structure can be well observed. 
He describes the whole sponge as furnished with a double 
envelope, the 66 outer and the inner parts of which are directly 
continuous into each other at many points.” 66 The outer division 
lies at a considerable distance from the monadigerous mass, and, 
as it were, suspended on the points of the larger or projecting 
spicules, just as a tent canvas is supported on the ends of poles. 
The inner division closely embraces the monadigerous mass 
like an epidermis, and even plunges beneath the hollow groups 
of monads, forming to them a basis of support In brief, 
we might say the sponge is covered with a miniature colonnade, 
whose ceiling is the outer division of the envelope, the pillars 
are the bundles of spicules, and the floor is tapestried by the 
inner division, which, about the pillars, hangs from the ceiling 
