238 



LOWER INVERTEBRATES. 



been described, connecting the various individuals of the colony, and although the 

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

 inter-communication exists, for there 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 lophophore. 



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 appen- 

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

 joint, and which, mo\ ed by appropriate muscles at the base. 



Fig. 233. — A portion ot Scntpo- , , , i i . j_' r^^^ • i ■ 



ceztaria /erox, with (a) vibra^ keep up a constant lashmg motion, llie avicularia, as is 

 ""'''■ partially indicated by their name, are shaped like th e head of 



a bird, with fixed ujiper 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, tluis rendering a colony of 

 some such form as liiKjidn a most interesting object under the 

 microscope. The purposes of those organs are as yet uncer- 

 tain. It has been suggested that tlie 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 prey thus caught to the mouth, 

 the part which tbey play in the nutrition of the polypide 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 tlie 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 separated from the parent, and form distinct indi- 

 viduals. Closely allied to this budding 

 process is the formation of statoblasts. 

 These are modified buds (and not true 



eggs), which are produced agamogenetically, the purpose of 

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

 species during the winter, or through a period 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 envelope, which in 

 many forms is ornamented by slender spines terminating with hooks, which more or 

 less vividly recall the flukes of an anchor. Late in the autumn the fresh-water polj'- 

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



Fig. 234. — Portion of Biif/u^a, 

 showing birds' heads or avic- 

 ularia. 



Fig. 235. — Statoblast. 



