948 FRESH-WATER BIOLOGY 



the substratum. The nature of the colony formation is variable: 

 sometimes close, forming a corm of zooids fused into one mass, as in 

 Cristatella; sometimes loose, each zooid being distinct, as in Urnatella. 

 Each zooid has a structure not unlike that of a rotifer. It 

 secretes a resistant outer covering. This is calcareous in some of 

 the marine forms, but is generally chitinous or gelatinous in those 

 of fresh water. So prominent and variable is this cuticula (con- 

 stituting the "zooecium," or "cell") that its form is frequently 

 used as a means of distinguishing species. The body wall is very 

 thin, having relinquished its protective function in favor of the 

 cuticula. It is separated from the viscera by a relatively enor- 

 mous body cavity. In the case of species whose zooids are fused 

 together the body cavities are confluent. The alimentary tract is 

 relatively simple. It consists of a U-shaped tube whose only 

 glands are localized in the epithelium of its walls. The mouth 

 end of the tube is furnished with a corona of numerous ciliated 

 tentacles which create a vortex at the mouth. The mouth is, in 

 one order of fresh- water species (Phylactolaemata), provided with 

 an "epistome" or protecting flap. A long esophagus leads to the 

 capacious stomach and this to the flask-shaped rectum. The 

 anus lies near the mouth either outside or inside the corona of 

 tentacles. For protection, the tentacles can be retracted quickly 

 under the shelter of the body cuticula like the proboscis of certain 

 polychaetes. There are numerous long slender muscles effecting 

 the retraction. The nervous system is simple. A brain lies between 

 esophagus and rectum and sends nerves to tentacles and alimentary 

 tract. Circulation is effected by the general fluid of the body 

 cavity. Well denned excretory tubules seem to be missing if we 

 except the doubtful case of certain Phylactolaemata. In the 

 Gymnolaemata the viscera periodically degenerate into a brownish 

 mass, a new alimentary tract regenerates, and the degenerated 

 mass passes through the wall of the gut and is expelled by the anus. 

 Eggs are formed on the body wall and, in Phylactolaemata, the 

 sperm on the mesentery (" funiculus ") . In the marine species the 

 embryos early become free-swimming, but in Phylactolaemata they 

 develop in a sort of uterus until they are young colonies. 1 These 



i The fresh-water form, Pectinatella magnified, produces a small free swimming spherical larva which 

 settles to the bottom almost immediately and there by budding gives rise to a colony. 



