MUSCLES OF THE BBYOZOA. 



273 



crown of gill-tentacles within the cell-orifice (Fig. 20, or) ; others having a like base, are fixed to the disc 

 or lophophore (/),and direct the rotary movements of the tentacular crown. Some dilate the flexible and 

 reversible portion of the cell (v), maintain the tentacular sheath in its right position when everted, 

 and withdraw it to again enfold the tentacles on their retirement. Not less important are those 

 muscles which control the immediate expansion of the tentacles, and their separate or combined 

 movements, or traversing the walls of the alimentary canal assist the processes of digestion. 

 According to the observations of various writers, the Moss-animal, when desirous of bringing its 

 crown of tentacles into communication with the water, in order to obtain the food and air 

 necessary for its existence, commences to erect itself by slightly straightening the alimentary tuba 

 The contraction of certain muscles called parietals, which permeate the mantle lining the cell, presses 

 the fluid contained in the perivisceral cavity, or space lying between cell wall and intestine, upon the 

 digestive organs. This pressure forces the tentacles to move upwards with the sheath, which is 

 therefore pushed beyond the cell orifice until, more or less completely everted, it is held in place by 

 its special muscles, and acts as a support to the alimentary organs of the little animal. The tips of 

 the closed tentacles are the first to appear, and on the relaxation of the controlling muscles are 

 rapidly unfolded and rotate with such speed and vigour as to create a perfect, if minute, whirlpool 

 in their immediate neighbourhood. Into the vortex thus created by the movements of the cilia the 

 small animalcules are engulfed, imprisoned within the circle of the ciliated breathing organs, and 

 finally carried to the mouth. Then, if found suitable, they are swallowed by the muscular 

 contraction of the walls of the gullet (ce), and pass into the stomach. When the nutrient particles 

 have been extracted the waste products are forced by the muscles of the digestive system into the 

 lower portion of the ciliated intestine, and finally ejected at the anal orifice. 



If sufficient food has been secured, or on the slightest alarm, the polypide is instantaneously 

 drawn within the protecting cell by the action of the great retractor muscles (Fig. '21, gr), the tentacles (t) 

 retire within the sheath (sh), which is again inverted, and its ridge closes the mouth of the cell in those 

 genera in which it is otherwise unprotected. All the inhabitants of the colony may be expanded at 

 once, and retreat one by one, or all at the same time, with the rapidity of lightning. Some are shy 

 in their habits, and, once disturbed, are long before they again emerge and venture to display their 

 glories ; while others stand rough treatment, and do not long keep within the retirement of their tiny 

 cellular habitations. 



The hollow tentacles external to the closed sac, which is perforated only by the openings for 

 the mouth and anus, are the most important structures in the Moss-animal 

 economy. They serve as respiratory agents, thus corresponding with the 

 gills of the Bivalve and the brachial appendages of the Brachiopod ; but 

 act also as organs of touch and prehension. They vary from eight to 

 eighty in number, and originate from the disc or stage, the lophophore,* 

 which roofs in the perivisceral cavity. As the lophophore is circular in 

 most of the marine forms, the gill tentacles present a bell-shaped appearance 

 (Fig. 18, c) ; but in many of the fresh- water genera it terminates in free 

 ends forming the crescent or horseshoe-shaped crown, whence the name of 

 Hippocrepian Bryozoa, often applied to this section of the class, is derived. 

 There is no trace of the existence of a heart or vascular system, and the 

 functions of circulation of the nutritive fluid are performed by means of the 

 cilia clothing the membranous body wall lining the cell ; circulation is, how- 

 ever, chiefly promoted by the muscular contractions of the mantle, which 



i_ . , 



keep up an incessant current and movement of the fluid filling the perivisceral 

 cavity. In this fluid the presence of white corpuscles has been detected. 



One of the most important structures connected, not only with the life-history of each individual 

 Moss-animal, but also with its relations to the rest of the colony, is that gelatinous tissue now known as 

 the endosarc;f this consists of a cord-like prolongation of an elastic membrane (the funiculus, Fig. 20, z), 

 which moors the base of the stomach to the bottom of the cell wall, and of divergent thread-like fibres, 

 which, passing through the pores in the communication plates existing between the different cells, 



* Greek, lophos, a crest ; phero, I carry. f Greek, endon, within ; sarx, flesh. 



225 



21. MOSS - ANIMAL 

 RETRACTED IN ITS CELL 



(After Hincks.) 

 tJi, Tentacular Sheath : gr, Great 



gus; z, Funiculus. 



