ANATOMY OF THE BEYOZOA. 



271 



opening, and is therefore within the disc or stage from which those breathing organs originate. Both 

 groups having the breathing organs or tentacles in a continuous series, are comprised in the Holo- 

 braiichiata.* A third and somewhat abnormal form, Rhabdopleura (Fig. 18, A), having a divided or 

 winged base for the tentacles which from their mobility and position present a close resemblance to 

 the brachial organs of the Brachiopocl at present constitutes the section Pterobranchiata. 



The Moss-animals, whether in the form of a minute shrub sprung from a creeping root-thread 

 (Fig. 16), or occurring in branched and riband-like encrusting masses, almost invariably consist 

 of a colony of individuals protected by a common external skeleton or ccencecium,t which 

 forms a defensive covering like the shell of a mollusc, differing, however, from that structure, 

 inasmuch as once formed it seems to have no further connection with the animal that originally 

 secreted it, surviving its death, and increasing independently. The assemblage of cells forming the 

 colony or polyzoarium is composed of no less than six distinct elements, all in reality of a truly 

 cellular nature, and 

 present in the higher 

 forms of the class. 

 (F. A. Smitt.) First 

 comes the true ani- 

 mal cell, the zooa- 

 cium, which lodges 

 a perfect Moss- 

 animal. Two other 

 kinds, devoid of 

 inhabitants, have 

 become metamor- 

 phosed into purely 



defensive organs ; ,-,. 



.' Fig. 18. A, WINGED; n, CRESCENTIC; c, CIRCULAR TYPE OF GILL TENTACLES, ENLARGED. 



while some, the Ovi- (After Hancock and Ray Lankester.) 



cells, are restricted 

 to reproductive pur- 

 poses. The stem-cells are simply elongated animal chambers ; and lastly, the radical-cells, or root- 

 fibres, which, sometimes hooked, act like little grapnels, mooring the colony to the soft organisms 

 to which it is attached, or, directed upwards, terminate in free, tendril-like claspers, enabling the 

 clinging animal parasite to gain a firmer foothold in the branches of the organism it infests. 



The animal cell is composed of two distinct integuments. The outer, or ectocyst,:}: a product 

 of the inner or endocyst forms the external cell wall, and is either entirely of a fleshy, gelatinous, 

 membranous, or of a membrane-gelatinous nature. In many cases it is partially encrusted with an 

 earthy, horny, or calcareous substance. The inner cellular membranous layer or endocyst lines the 

 outer wall. It always remains soft, transparent, is contractile, and corresponds with the mantle of the 

 mollusc, and forms the body-wall of the animal. In the fresh-water genera its inner surface is clothed 

 with vibratile cilia. The endocyst terminates in a fold near the horny ring or lophophore (Fig. 18, I), 

 from which the tentacles (t) surrounding the mouth originate. Usually credited with the 

 functions of the enlargement of the colon} 1 ", it is now also associated with the production of a third 

 element a tissue, the true nature of which has only recently been determined, as one highly important 

 to the general welfare of the community. The colony is in all cases derived from the metamorphosis 

 on fixation of a single free-swimming larva, which develops into the primary Moss-animal, whence the 

 colonial aggregation subsequently arises by a varied process of indefinite repetition. 



Each of the animal cells forms the home of a minute, but separate, animal, often termed the 

 polypide, which, in the majority of species, possesses all the organs requisite for the exercise of its 

 individual, nutritive, and reproductive functions. It lives an independent existence in the cell or 

 chamber it inhabits, to the inner wall of which it is usually attached by muscles, which enable its 

 alimentary organs to partially protrude from, and retire within, its cell at its own pleasure. This 



* Greek, holos, entire; Iranchia, gill. t Greek, koinos, common; oikos, house. 



J Greek, kustis, a sac or bladder. 



/, Lophophore ; i.Tentacles; m, Mouth ; o, Anus; g, Nerve ganglion; e, Epistomc or foot ' /, Divided or Winged 



base of KhuMoyleura. 



