202 NATURAL HISTORY OP 



species are distinguished from the marine species by 

 possessing- a valve-like organ (the ejjistome) arching 

 over the mouth. The members of the entire class are 

 fixedj with the exception of Oristatella, a fresh-water 

 genus, which is locomotive. 



Each animal in a colony of Polyzoa consists of two 

 parts — a zooecium or cell and a polypide — which two 

 parts are vitally connected, the zooecium being not 

 merely an unorganized protection for the polyp, as in 

 the case of the Hjdroida, but being itself in organic 

 connection with the polypide. Indeed it is sometimes 

 argued that the zooecium is a permanent organism, and 

 the contained zooid an ephemeral one. This argument 

 is based upon the fact that the zocecium may continue 

 alive after the destruction of the polypide, and may 

 have succesi^ive new inmates. Many naturalists, how- 

 ever, consider the zooecium and polypide as forming 

 together one animal or polyzoon. 



The general appearance of a polyzoon may be seen 

 from Plate XVI., fig. 1. The zooecium is a membra- 

 nous sac (c), often strengthened by a deposit of calca- 

 reous or silicious matter, and lined by the endocyst, 

 vs^hich extends over the orifice of the zooecium in order 

 to form a sheath for the tentacles. The zooecium is, 

 therefore, closed by the endocyst except in two places, 

 one the anal orifice, and the other the aperture or orifice 

 through which the tentacles protrude ; but as the 

 protrusion of the tentacles is simply like the puUing 

 out of the finger of a glove, there is no real opening 

 out of the zooecium, which is eutii-ely sealed up. The 

 endocyst, or internal lining of the zooecium, forms the 

 sheath of the tentacles {t), which are upon a stage 

 called the lo^jhoi^hore {I). Between the tentacles is the 



