IV. POLTZOA: ECTOPROCTA. 323 



entocyst is always covered by the ectocyst only on the basis and side 

 walls; the peripheral end remains soft and forms a sort of collar 

 into which the tentacles and adjacent parts of the cystid can 

 be retracted. In the ectocyst there is, as will be seen, a larger or 

 smaller opening which in many species (Chilostomata) can be 

 closed by a lid (operculum). The circle of tentacles surrounds 

 the mouth alone, while the anus is outside near the collar. The 

 strongly bent alimentary canal extends into the cystid and is bound 

 at its hinder end by a cord, the funiculus, to the base of the cystid. 

 Ganglion and nephridia lie between the mouth and anus. The 

 gonads arise from the epithelium of the coelom, the testes usually 

 on the funiculus, the ovaries on the wall of the cystid. 



Hundreds and thousands of individuals form colonies (fig. 297) 

 in which cystid abuts against cystid. The coelom of adjacent cystids 

 may be distinct or a wide communication may exist. The colonies 

 grow by budding; in the Gymnolaemata a part of a cystid becomes 

 cut off as a daughter cystid in which the polypid alimentary tract 

 and tentacles arises by new formation ; or (Phylactolaemata) the bud 

 anlage of the polypid arises before the first appearance of the cystid. 



Division of labor or polymorphism is common. Besides the 

 animals already described, which are primarily for nourishment, 

 three other individuals may occur, ovicells, vibracularia, and avic- 

 ularia. All three are cystids which have lost the polypid. The 

 ovicells are round capsules which serve as receptacles for the 

 fertilized eggs. The vibracularia (fig. 295, B) are long tactile 

 threads; the avicularia (fig. 295, A) are grasping structures of 

 uncertain function. They have been seen to seize small animals 

 and hold them until decay set in. It is possible that the fragments- 

 serve as food for the polypids. The avicularia have the form of a, 

 bird's head, the movable lower jaw being a modified operculum. 



Under unfavorable conditions a polypid in a cystid may break down, 

 and be lacking for some time until better relations cause its new forma- 

 tion. Besides in the depopulated cystids there may appear statoblasts, 

 lens-shaped many-celled internal buds enveloped in a firm envelope which 

 form a resting stage for the preservation and distribution of the species. 

 Each statoblast is surrounded by a girdle of chambers which by drying 

 become filled with air, causing the statoblast to float when it again comes, 

 into water. From the statoblast a smaller polyzoon escapes which de- 

 velops a new colony. The statoblasts are adaptations to the conditions 

 of fresh-water life and occur only in the Phylactolaemata. 



Order I. Gymnolaemata (Stelmatapoda). 



The tentacles in a ring around the mouth. The numerous species are 

 almost exclusively marine and are abundant on every coast. In the 



