242 CONTRIBUTIONS TO THE NATURAL HISTORY 



of muscular tube, in which, however, the tissues do not assume a membranous appear- 

 ance properly, but rather preserve their cellular nature. On the inner surface, we find 

 another layer of epithelial cells, i, i. The contraction of the cells of the middle layer 

 can be seen so easily under the microscope, that, if any doubts should be entertained 

 upon the question of cells, as such, being organs of locomotion, I would simply refer to 

 an investigation of this proboscis. There are, however, some peculiarities about which 

 I am not prepared to give a decided opinion. For instance, are the threads which extend 

 between these contractile cells contractile fibres or sensitive fibres ? As for the walls 

 of the central cavity and the radiating vascular tubes, as well as the circular tubes below, 

 they seem structureless ; that is to say, the elements of which they consist cannot be 

 traced beyond the main appearance, which is that of very thin membranes, and whether 

 they are intercellular spaces lined with solidified gelatinous matter, or whether they are 

 particular rows of cells, confluent into each other so as to form tubes, I cannot deter- 

 mine at present. But so much is true, — that the whole system of these peripheric 

 tubes communicates in all its parts, and that the movement of the fluid within is not 

 owing to the contraction of the walls of the circulating tubes, but is produced by the 

 contractions of the proboscis, in which there is a distinct muscular layer, and by the 

 general changes of form arising from the contractions of the voluntary muscles, and also 

 from the contractions of the tentacles, which, by their base, are connected with the 

 circulating apparatus. And although we have here a regular alimentary canal, and 

 this canal branches off and assumes the appearance of a circulatory system, we have prop- 

 erly neither a digestive system, in the sense in which we acknowledge it in other animals, 

 nor a circulatory apparatus ; for there is no distinction between the alimentary canal 

 proper and the vascular system, the one opening through large tubes into the other ; 

 nor is there a distinct circulatory apparatus, as the fluid circulated in these tubes con- 

 sists of the contents of the stomach which have been emptied into them. Therefore 

 we should be on our guard against comparisons involving identity of plan and of struc- 

 ture, when tracing the analogies or homologies between these parts. As far as I am 

 concerned, I would prefer to call the central part of this system the proboscis, rather 

 than to call it the mouth and stomach ; I would prefer to call the narrow cavity above, 

 the central, circulatory cavity, rather than to call it the heart, and I would prefer calling 

 the vessels arising from it chymiferous tubes, to calling them either bloodvessels, or 

 circulatory vessels, or alimentary vessels. 



Another fact, which shows how little, — I will not say, homology, — but how little 

 analogy, there is between such an arrangement of tubes and that of the vessels of higher 

 animals, is this, that the sensitive swelling, — as every body will allow the eye-speck with 



