OF THE ACAI,EPH.E OF NORTH AMERICA. 241 



eructated, or at least pressed out into the lower end of the proboscis, where the mouth, 

 contracting closely, causes the proboscis to swell above, without allowing the food to 

 escape, which may gradually retrace its way into the central cavity, then be circulated 

 into the radiating tubes, reach the lower circular canal, and move about in it, sometimes 

 advancing from the centre towards the periphery, at other times rising from the pe- 

 riphery towards the centre, and moving alternately one way or the other in the circular 

 tube. There can be no doubt as to the irregularity of these movements, as the globules 

 or granules suspended in the more liquid food will enable any one, even with a low power, 

 to trace the course of the nourishing fluid. 



The tentacles, also four in number (Plate IV. Figs. 1,2; Plate V. Figs. 2, 4, 11,/, /, /, /), 

 arise from the lower margin of the disk just at the points where the vertical tubes unite 

 with the circular canal, and at these points we notice a sort of bulb (Plate V. Fig. 11, 

 m, m), consisting of the swelling of the base of the tentacle in its connection with the 

 chymiferous tubes, and also of a peculiar accumulation of cells, forming a rudimentary 

 visual apparatus in the form of black eye-specks. (Plate V. Fig. 13, a, and Fig. 14, 6 ; 

 also, Fig. 7, i, and Fig. 12, d, &c.) 



The presence of nourishment within the chymiferous system renders it more or less 

 apparent to the naked eye. There is, however, hardly any state in this animal, in which 

 the cross formed by the four tubular radii in their connection with the small narrow 

 cavity and proboscis could not be distinguished, even with the naked eye. I mav as 

 well mention at once, that the cords of sensitive cells (Plate IV. Fig. 1,2,3; Plate 

 V. Fig. 2, 4, 11) accompany everywhere the chymiferous tubes, so that the tracts which 

 are made prominent to our senses by the color of the food circulated through these 

 tubes are the most important points in the structure of these animals. The chymiferous 

 system, in connection with the nervous system, marks, as it were, the most prominent 

 feature of an animal, in which the muscular system alternates in the arrangement of its 

 essential parts with these tracts. 



The intimate structure of the digestive and circulatory apparatus is as follows. The 

 proboscis consists of a tube, in which we may distinguish three layers of cells. There 

 is an outer or epithelial layer, under which the eggs are developed, when the animal is 

 mature. No eggs, however, are formed about the mouth, or in the uppermost part of 

 the proboscis ; the middle region of its tract alone is covered with crowded eggs, and to 

 such an extent, that, at the spawning season, the proboscis (Plate V. Fig. 7, b) seems 

 much thicker than under ordinary circumstances. The next layer consists of contractile 

 cells ; it is the most powerful layer, it is also thicker, and extends, in unbroken continuity, 

 from the margin of the mouth to the central cavity. (Plate V. Fig. 8, e, e.) It is a kind 



