316 ZOOLOGY SECT. 



The body-wall consists, in addition to the cuticle, of an epidermis 

 composed of a single layer of large flattened cells, two muscular 

 layers, the outer circular and the inner longitudinal, and a layer of 

 an irregular cellular tissue, or parenchyma. 



The coelome is extensive ; it is lined externally by the parietal 

 layer of parenchyma forming the innermost layer of the body- wall, 

 and internally by a visceral layer of the same tissue ensheathing 

 the alimentary canal. Across the cavity between the parietal and 

 visceral layers of the parenchyma pass numerous strands of spindle- 

 shaped cells. A large double strand (funic) passes from the 

 proximal or aboral end of the alimentary canal to the aboral 

 wall of the zocecium ; this is the funiculus. The coelomic fluid 

 contains a number of colourless corpuscles or leucocytes. 



Alimentary Canal. The mouth (mo) leads into a wide chamber 

 the pharynx (pli) just behind the bases of the tentacles ; from 

 this a somewhat narrower short tube, separated by a constriction 

 from the pharynx, leads to the stomach (stom) from which it is 

 also separated by a constriction. The stomach gives off a long 

 conical prolongation or ccecum passing towards the aboral end of 

 the zocecium, to which it is attached by the funiculus. The 

 intestine (int) comes off from the oral aspect of the stomach, not 

 far from the oesophagus, with which it lies nearly parallel : it ter- 

 minates in a rounded anal aperture (an) capable of being dis- 

 tended to a considerable size, situated not far from the mouth, 

 but outside the lophophore. The entire alimentary canal is lined 

 by an epithelium, which is ciliated throughout except in a portion 

 of the stomach : the cells of the epithelium, which are arranged 

 in a single layer, vary in length in different regions, being longest 

 in the pharynx, which is comparatively thick-walled. A pair of 

 slender muscles (gast) passing from the body-wall to the stomach 

 act as retractors of the alimentary canal when the introvert is 

 drawn back. 



There are no blood-vessels. 



A nervous system has not been traced in Bugula avicularia : 

 but in many other Polyzoa a small rounded ganglion is distinguish- 

 able between the mouth and the anus; giving off nerves to the 

 various parts ; organs of special sense are absent. Definite ex- 

 cretory organs do not occur in Bugula, the function of excretion 

 (i.e. the collection of the nitrogenous waste-matters) being appar- 

 ently carried on by the leucocytes and the cells of the funicular 

 tissue. 



Reproductive Organs. Ovary and testis are found to occur 

 together in the same zooid. They are both formed from specially 

 modified cells of the parenchyma, either of the funiculus or of 

 the body-wall. The testis, developed from the cells of the funicular 

 tissue, gives origin to spherical masses of cells the spermatidia 

 (sp) which develop into sperms with very long motile tails. These 



