238 



LO WEIi IN VER TEBRA TES. 



of Srrupo' 

 , with (fi) vibra- 



bi'cn described, connecting the various individuals of the colony, and altliough the 

 nervous nature of these cords has been disputed, it is evident that some means of 

 inter-communieation exists, for lliere is frequently such a 

 unison in the movements of the various members of a stock 

 that no other explanation is possible. Nothing definite is 

 known of the organs of sensation. The muscular system is 

 well developed, the most prominent portions being the re- 

 tractors and protractors of the lophojihore. 



Possibly the structures known as avicularia and vibracula 

 are the most interesting to the layman, on account of their 

 motions and problematical functions. These organs are not 

 found in all forms. The vibracula are long, whip-like ap]ien- 

 dnges, which are attached to the cells of the colony by a single 

 joint, ;uid which, moved by appropriate muscles at the base, 

 keep u}i a constant lashing motion. The avicularia, as is 

 partially indicated by their name, are shaped like the iiead of 

 a Iiird, with hxed ujij)er and movable lower mandibles. These 

 avicularia are either directly attached to the cell, or are ele- 

 vated on a short stalk, and, in life, keep in constant motion, 

 opening and closing the mandibles, tlius rendering a colony of 

 some such form as liugula a most interesting object under the 

 rnicroscoi)e. The purposes of these organs are as yet uncer- 

 tain. It has been suggested that the constant lashing of the 

 vibracula serves to clean foreign matter from the colony. The 

 avicularia are frequently seen to seize small aquatic objects, 

 but as they cannot carry the i)rey thus caught to the mouth, 

 the ])art which they play in the nutrition of the ])olypide is 

 at least indirect. Mr. Gosse, the entertaining English writer 

 on natural history, has suggested, with considerable plausi- 

 bility, that the decay of the objects caught by the avicularia 

 attracts other organisms to the vicinity, thus bringing them 

 within the influence of the currents produced by the cilia on 

 the tentacles, and thus to the mouth. 



The Polyzoa reproduce both by budding and by eggs. 



Usually the buds remain attached to the parent stock, thus 



causing it to increase in size ; but in one or two forms the buds 



become sejiarated from the parent, and form distinct indi- 



^ — ^^ viduals. Closely allied to this budding 



' 3'^^^^^^^'^°* process is the formation of statoblasts. 



Fig '35 — sfutobiasi ' hese are modified buds (and not true 



eggs), which are produced ag.amogenetically, the purpose of 

 which, like the statoblasts of sponges previously described, is to jierpetuate the 

 species during the winter, or through a j)eriod of dry weather. The statoblasts arise 

 from the funiculus, or cord which connects the stomach with the cell. As they 

 increase in size, they become invested with a thick, horny brown envelojie, which in 

 many forms is ornamented by slender s])ines terminating with hooks, which more or 

 less vividly recall the flukes of an anchor. Late in the autumn the fresh-w.ater poly- 

 zoan dies, and then the statoblasts are set free to perpetuate the colony in the following 



•'IG. 2.14.— Portion of liiniu'a. 

 showing birds' lieads or avic- 

 ularia. 



