ON RECENT POLYZOA. 483 
The peristome may be— 
1. Thin or thick. either rigid or articulated. 
2. Elevated, produced, or bevel. 4, Unarmed. 
3. Armed with oral spines, 
The other divisions of this part of Mr. Busk’s work refer to :— 
III. Special pores on the front of the zocecia. 
IV. The Ocecia. 
V. Special organs: a. Avicularia, 
§ as to function. | §§ as to position. 
b. Vibracula: a. simple, 6. compound. 
VI. Chitinous elements. 
Opesia, Dr. Jullien. Certain of the Membranipore having a thick 
calcareous expansion were covered when living by a membrane, in which 
was the opercular opening. This is the ‘opesia’ or opesial opening of 
Dr. Jullien. 
Aperture—restricted for the proper opening. 
General Plan of Structure. 
_ *In every polyzoon,’ says Mr. Hincks, ‘we distinguish two fynda- 
mental elements, the zocecium and the polypide. These are the primary 
MS, membranous sac ; ¢, tentacular crown; 0, mouth; es, esophagus ; 
Ss, stomach ; a, anus; int, intestines; g, ganglion; 7, funiculus ; 
ov, Ovary ; Sp, spermary. (After Hincks). 
: zooids in every colony. When the larva fixes itself, after its brief term 
_ Of free life, it is metamorphosed into asingle sac or cell, enclosing a mass 
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