90 UNDERWATER GUIDE TO MARINE LIFE 



There are a "reat number of marine species, none of which has a skeleton. They 

 are not common as oceanic plankton, but they are very numerous inshore in 

 shallow water or as parasites. The species of ciliates live wholly by hunting 

 down prey or by filtering sea water by means of currents set up around their 

 "mouths" by the cilia. Because of these active modes of feeding, ciliates are 

 the most active of protozoans and show more response to environment than do 

 other protozoans. 



One group of ciliate derivatives called "suctorians" has lost cilia and has 

 acquired long tentacles by which prey is captured and sucked dry. Prey usually 

 consists of other protozoans or small larvae of invertebrates and may be several 

 times the size of its captor. Suctorians are very common attached to substrate 

 or to other animals in shallow waters. 



SPONGES: Phylum Porifera {"pore-bearer") 



Colonial animals are those in which several individuals have banded together 

 to form one functional unit. Multicellular animals are those in which the 

 different cells have become associated into different organs for the performance 

 of different jobs. Sponges sit on the fence between being colonial and multi- 

 cellular. They probably arose from flagellate protozoans because they are 

 composed of aggregates of flagellated cells, but these cells are not gathered 

 together to form organs. There are several types of cells, some wandering and 

 amoebalike, and there is a supporting skeleton. Therefore, sponges, though 

 they are not on the direct evolutionary line toward multicellular animals, do 

 show how multicellular animals might have arisen through associations or 

 colonies of single cells. 



The partial independence of the cells of sponges is demonstrated by the 

 sponges' powers of regeneration. Sponges are a bit like the magical broom 

 of "The Sorcerer's Apprentice." If a sponge is cut into many pieces, each piece 

 is capable of producing a new sponge. It is even possible to separate the cells 

 of a sponge by squeezing it through a fine cloth. These cells, if they are still 

 living, will group together again and grow into a new sponge. 



Sponges are very plantlike and, in fact, were not known to be true animals 

 until the nineteenth century. Basically, a sponge is a hollow vase perforated 

 by holes (hence the name "pore-bearer"). There are many small holes by which 

 water enters the vase and one larger hole, or a few larger holes, bv which water 

 leaves. The latter are large enough to be easily seen by the naked eye. Currents 

 of water through the vase are caused by the beating of the flagella of the cells 

 on the inside and bring microscopic, planktonic food to the cells of the sponge. 

 In most cases, sponges are not merely simple vases, however, but have become 

 much infolded and complicated in form, so that the internal surface area 

 of the sponge is greatly increased without involving much of an increase in size. 



Sponges have skeletons composed of calcium carbonate, silica compounds, 

 or spongin fibers. The first two are brittle and in the form of delicate, ncedlelikc, 

 branching spicules which are enmeshed together to form the sponge's supporting 

 structure. Spongin is resilient and is the substance known to us all as the fibers 

 of the bath sponge, Eiispongia. It is largely on the basis of their skeletons that 

 sponges are classified. 



