28 SUMMARY OF CURRENT RESEARCHES RELATING TO 



these there are pores by which the mucous secretion passes to the 

 exterior. The cylindrical cells have an important function in the move- 

 ment of the colony, as they secrete a thin transparent and chitinous 

 membrane, which affords a smooth surface which lessens friction and 

 affords a strong fulcrum. The musculature of the foot consists of a 

 longitudinal and a transverse layer, the fibres of which are set at right 

 angles to one another. The cells of the mesodermal epithelial layer are 

 provided with very short cilia which can be easily missed, and which the 

 author only saw with certainty in living specimens. The septa which 

 traverse the cavity of the pedal disc are completely formed of mesoderm ; 

 they are made up of a hyaline supporting membrane on either side of 

 which are longitudinal fibres and pavement epithelium ; their layer of 

 transverse fibres is feebly developed or completely wanting. 



The integument of the separate individuals is the direct continuation 

 of the upper covering of the disc, and consequently consists of the same 

 three layers as compose it ; although there are, of course, certain differ- 

 ences in the details. The walls of the lophophore and of the tentacular 

 crown are formed of the same layers as the cystid. The tentacles are to 

 be regarded as evaginations of the cavity of the lophophore, which, again, 

 communicates with the body-cavity. 



The enteric tract is made up of the endodermal enteric epithelium, a 

 median muscular layer, and an outer mesodermal ccelomic epithelium. 

 The epistome carries externally a layer of ciliated cells, which are 

 highest near its base. The foregut is divisible into two parts, which are 

 histologically quite distinct. The lining epithelium of the pharynx is 

 the direct continuation of the ciliated investment of the epistome, and 

 presents very long, delicate, ciliated cells, separated from one another, 

 like those of the epistome, by clefts. About the middle of the foregut 

 the ciliated cells suddenly cease, and the epithelium of the oesophagus 

 commences. Its cells are long, delicate, and cylindrical, but they have 

 no cilia, and do not stain like those of the pharynx ; nor are they sepa- 

 rated from one another by clefts. Inferiorly, the foregut is bounded 

 by a circular valve, which at its margin takes on the characters of the 

 epithelium of the stomach. As in Alcyonella, the stellate form of a trans- 

 verse section of the lumen of the stomach is due to the fact that the cells 

 which form the longitudinal ridges of its wall are knobbed at their free 

 ends and greatly elongated, while the intermediate cells have sharper 

 tips and are comparatively short. It will be observed that there is no 

 formation of true folds, but it is of greater interest to note that the cells 

 and the ridges are histologically and physiologically different from those 

 which are found in the intermediate grooves. The former have generally 

 one or two thin transverse walls which appear to be formed by hardened 

 secreted surf ices ; the grey finely granular contents at the knobbed end 

 are much darker than those of the rest of the cell, and often, indeed, 

 the upper cell-boundary is quite broken through by the finely granular 

 secretion which passes freely into the stomach. The secretion of the 

 ridge-cells does not stain, while those of the groove-cells always take a 

 dark colour throughout their whole extent ; that of the former is a slimy 

 mass which envelopes the particles of the food and connects them with 

 one another. 



The rectum is sharply distinguished from the stomach ; the contour 

 of its lumen is round, and its lining cells low and broad. 



The mechanism of digestion has been observed in living specimens 



