248 Dr, T. Williams on the Mechanism of Aquatic 



manner than the stomach, with the channel of the stem. In these 

 families, the tentacles, which are evidently the continuations of 

 the pevigasti-ic chambers (fig. 1, a), are said to be perforate at 

 their distal extremities : this point requires to be confirmed. The 

 fluid filling the axial channel of the stem enters at the mouth of 

 each polype, and descends through the orifice situated at the base 

 of the stomach (fig. 1, c) into the polypidom. In this situation, 

 in a great variety of species, the motion of the corpuscles con- 

 tained in the fluid may be readily observed. They present all 

 the characters of being driven by cilia. The presence of cilia is 

 however controverted by some observers. From the polypidom 

 the fluid passes upwards into the perigastric chambers, and thence 

 into the tentacles in which it undergoes aei'ation. It is curious 

 that the corpuscles of the fluid of the stem do not pass upwards 

 into the tentacles. They are filtered back by the cribriform 

 partition, which divides the chambers around the stomach from 

 the axis of the polypidom. New observations ai'e required 

 on the whole family of the asteroid polypes, having special re- 

 ference, 1st, to the arrangement and existence of cilia, and 2nd, 

 to the distribution of the fluids. 



The fluid by which the whole extent of the stem and visceral 

 chambers of each individual polype are distended, constitutes 

 one system. So rapidly is this fluid endowed with a low order 

 of vital properties, enabling it to fulfil its functions as an element 

 of nutrition, that it may be rejected en masse, to be replaced with 

 a fresh volume of zraorganic water. Such is the converting power 

 of the vital chemistry in these simple organisms. This fact 

 distinguishes the polype families from all other invertebrate 

 animals. The true character of the breathing function must 

 have remained beyond the reach of the physiologist, without the 

 knowledge of these points. They prove that the lowei- the vital 

 endowments of the fluids, the simpler the mechanical arrange- 

 ments required to efiect their aeration. In zoophytes the nutri- 

 tive fluid is not exclusively vitalized through the agency of floating 

 cells, it is vivified in part, catalytically by contact with the sur- 

 faces of the living solids. The moq^hotic elements, therefore, 

 which exist in the fluids of this group, are scanty in number, sub- 

 ordinate in function, and indeterminately organized. To detect 

 the globules in the fluid of the polypary is easy. It is more dif- 

 ficult to trace its progress upwards into the space which suiTounds 

 the stomach, and thence into the tentacles. If, as lately stated by 

 Prof. Allman*, the axes of the tentacles in the tubularian polypes 

 open directly into the stomach, and not into the space to the outside 

 of this organ, these appendages cannot be intended to expose the 



* Proceedings of the Royal Society, May 31, 1853. 



