122 STRUCTURE OF TUNICATA AND POLYZOA. 



are covered with cilia, by the action of which a continual 

 stream is made to flow over the gills and to enter the stomach ; 

 and the minute particles which the water brings with it, and 

 which are adapted to serve as food, are retained and digested 

 in the stomach. Even these animals, fixed to one spot during 

 all but the early part of their lives, and presenting but very 

 slight indications of sensibility, possess a regular heart and 

 system of vessels ; and these vessels form part of the stem, t, 

 by which the compound species are connected. A single 

 nervous ganglion is found between the two orifices ; this 

 seems to receive sensory fibres from tentacula situated around 

 the oral orifice, and to transmit motor filaments to the mus- 

 cular coat which underlies the outer tunic, so that any irrita- 

 tion applied to the former occasions a contraction of the 

 latter, which tends to expel the offending particle. This 

 class is one of particular interest to the naturalist, since we 

 see in it the tendency to the formation of compound struc- 

 tures, by a process resembling that of the budding of plants, 

 which is essentially characteristic of Zoophytes; this ten- 

 dency, however, is more fully manifested in the succeeding 

 class. 



115. The animals forming the class POLYZOA (more com- 

 monly known as Bryozoa) are seldom or never found solitary ; 

 since, in consequence of their universal tendency to multiply 

 by gemmation, they form clusters or colonies of various kinds. 

 The body of each individual is inclosed in a sheath or " cell," 

 which is sometimes horny, sometimes calcareous ; and the 

 composite skeleton formed by the aggregation of these, which 

 has sometimes a branching or leaf-like form, but sometimes 

 possess the compactness of a stony coral, is known as the 

 " polyzoary." In their general structure the animals of this 

 class possess considerable analogy to the Tunicata ; but the 

 Molluscan type presents itself under a more degraded aspect, 

 no vestige of a heart or of blood-vessels being here dis- 

 cernible, and the general structure being so simplified as to 

 manifest no great degree of elevation above that of Polypes. 

 The typical structure of these animals may be understood 

 from that of the Eowerbankia (fig. 64), which is one of those 

 whose cells are not in contact with each other, but grow forth 

 at intervals from a creeping stem. The mouth, a, is situated 

 in the midst of a circle of arms fringed with cilia; these 



