POLYZOA. 



239 



spring. Then, nnder the influence of the warmth, the statoblast hatches with its 

 organs ah-eady well developed. At first it swims freely tlirough the water, but soon 

 it becomes attached to some submerged object, with which henceforth its fortunes are 

 inseparably united. It now develops its capsule, and soon a bud is seen upon one side, 

 which eventually grows into an individual, undistinguishable from the parent. This 

 process is again and again repeated, until a large colony is formed, either extending 

 its branches like a tree, or incrusting some submerged object with a gelatinous or cal- 

 careous envelope, forming in some instances clusters several feet in diameter and 

 eight or more instances in thickness. This same process .srScJ^^ 



of budding takes place in the marine genera. > * ^ww.»i»" 



We do not yet know enough about the development 

 of the eggs of the Polyzoa to reconcile all the widely dif- 

 ferent features of the embryology. Still, all the various 

 forms of lai-vse may be reduced to a body surrounded by 

 a ring of cilia (possibly corresponding to the lophophore), 

 which divides it into two faces. On one of these is the mouth, and in some the anus 

 also. On the other is a ciliated disc, by which, it may be, the animal attaches itself. 



The Polyzoa first appear in time in the silurian rocks, and have persisted to the 

 present day. The oldest forms known are referable to groups now living. At one 

 time it was thought that the graptolites might belong here, but now the best authori- 

 ties are inclined to place them among the hydroids 



Fig. 236. — Larva of Alcyonidium^ 



Sub-Class I. — Entopeocta. 



The primary feature characterizing this group is the position of the vent, which is 

 placed within the circle of tentacles, thus indicating that the group is the lowest of 

 the sub-classes, a feature which is characteristic of the larvae of some of the higher 

 groups here persisting in the adult, as can be seen by comparing the figure of Cypho^ 



nautes (fig. 246), which is the larva of Membrani- 

 pora, with that illustrating the anatomy of the adult 

 Pedieellina (fig. 237). In this grouj) the tentacles 

 are not retractile, but can be rolled up. 



In the Pedicellinid^ there is a creeping root- 

 stock, from which at intervals the long-stalked indi- 

 viduals arise and the colony increases by budding. 

 On our coasts is found Pedieellina americana, which 

 creeps over other Polyzoa, hydroids, etc., forming 

 small white branching stems, the stalked individuals 

 resembling so many clubs. Urnatella is a fresh- 

 water genus, represented, so far as is at present 

 known, by only a single species, Urnatella gracilis, 

 from the Schuylkill River at Philadelphia. In this 

 brightly colored form the cells are borne on the ex- 

 tremities of long noded branching stalks. In former years the species was very abun- 

 dant just below the dam which supplies the city of Philadelphia with water, but now, 

 doubtless owing to the pollution of the river by sewage, specimens are but rarely 

 found. 



The LoxosoMiD^, which were first made known in 1863, are long-stalked, solitary 



Fig. 237. — Anatomy of Pedieellina ; h, 

 blood pouoli; m, moiitb; n, ganglion; r, 

 rectum; s, stomacli; t, tentacles. 



