THE BRYOZOA — BASSLER. 341 



they form open-meshed lacework of the most regular and beautiful 

 patterns. Most bryozoa are attached either basally or by the greater 

 part of their surface to extraneous objects or are moored to the bot- 

 tom by rootlike appendages. In, many forms the zoarium is regu- 

 larly jointed to give greater mobility. 



The individual zooids of the zoarium conform to a simple and 

 definite type of structure throughout the class. The soft parts of the 

 animal consist of an alimentary canal with three distinct regions 

 discernible — esophagus, stomach, and intestine. The alimentary 

 canal is inclosed in a sac and bent upon itself so that the two ex- 

 tremities are close to each other. The mouth, or oral opening, is 

 either entirely or partially surrounded by a row of slender, hollow, 

 ciliated tentacles which serve for respiration and for sweeping food 

 toward the mouth. The two large divisions under which the bryozoa 

 are classed (Entoprocta and Ectoprocta) are based upon the position 

 of the anal opening. In most cases the anal opening is situated with- 

 out the row of tentacles (Ectoprocta) ; rarely it is placed within, 

 this row (Entoprocta). A heart and vascular system are wanting, 

 but there are numerous leucocytes floating in the general cavity. A 

 nervous ganglion is present between the mouth and anus and sends 

 delicate nerve filaments to the tentacles and esophagus. The upper 

 part of the sac is generally flexible and can be invaginated through 

 the action, of numerous longitudinal and transverse muscles which 

 traverse the fluid-filled visceral cavity. 



The reproductive organs are developed in various parts of the 

 body cavity, although the spermatozoa occur usually in the lower 

 and the ova in the upper part. The ova may be developed in a 

 special receptacle, in an inflation of the surface, or in a modified 

 zooecium. The general term ooecium or ovicell is applied to all 

 of these structures. 



The above general description of the anatomy of the bryozoan 

 applies, with certain exceptions, to all divisions of the class, and 

 more modification in structure is to be observed in the protective 

 covering or home of the animal, the zooecium, than in the animal 

 itself, the polypide. The accompanying diagram of the anatomy 

 of a single zooecium (text fig. 1), with its polypide retracted, will 

 illustrate this general structure. The mouth leads into the ciliated 

 pharynx, and this into the esophagus, followed by the stomach, 

 which in turn passes into the intestine, and this through the rectum, 

 communicates with the exterior by the anus. When retracted the 

 tentacles lie in a cavity, the tentacle sheath, which opens to the 

 exterior by the orifice. 



Many bryozoans exhibit, attached to the zooecium, organs re- 

 sembling a bird's head, termed "avicularia," and other bristlelike 

 appendages named "vibracula." The jaws of the avicularia open 



