vni PHYLUM MOLLUSCOTDA 343 



tactile, and also act as organs of respiration. When retracted they 

 become enclosed by the walls of the introvert as by a sheath — 

 the tentacle-sheath. A pair of bands of muscular fibres — the parieto- 

 vaginal muscles (ret.) — passing to the introvert from the body- wall, 

 serve to retract the introvert and tentacles. 



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 zooecium ; this is the funiculus. A transverse partition 

 cuts off (though not completely) a small anterior compartment of 

 the coelome from the rest. The former surrounds the basis of 

 the tentacles, the narrow internal cavities of which are in com- 

 munication with it : this is known as the circular canal. The 

 coelomic fluid contains a number of colourless corpuscles or 

 leucocytes. 



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

 chamber — the pharynx (ph) — just behind the bases of the tentacles ; 

 from this a somewhat narrower short tube, separated by a constric- 

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

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

 conical prolongation or cozcum passing towards the aboral end of 

 the zooecium, 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. 



The nervous system consists of a small round ganglion situated 

 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 



