330 



sive act, and that the apparatus just mentioned is amply 

 sufficient for the purpose, without having recourse to the 

 agency of the bent oesophagus, — an agency which in pluma- 

 tella repens, and other fresh water zoophytes, which I have 

 examined, assuredly does not exist, as in these the CESophagus 

 is straight during the retracted state of the animal. In the 

 remarks now offered, however, I do not deny that some in- 

 fluence is exercised by the straightening of the cesophagus 

 in those species in which this tube is bent upon itself pre- 

 viously to the commencement of protrusion ; I merely wish 

 to assert that we are not necessarily obliged to have recourse 

 to this agency, but that in some instances, at least, the pa- 

 rietal muscles, or, in their absence, the general contractility 

 of the internal tunic, are agents perfectly effective. 



" Such are the observations which I have had an oppor- 

 tunity of making on the muscular system of the ascidian 

 polypes of fresh water. I am fully aware that they are far 

 from being perfect, but for this my excuse must be found in 

 the extreme difficulty of such investigations, made, as they 

 all necessarily are, under a high power of the microscope, 

 with an illumination most carefully adjusted, and which can 

 by no means be at all times obtained, in order that structures 

 of such extreme tenuity and transparency may not entirely 

 escape detection. The observations have all been made on 

 the living animals, and no one who has not devoted himself 

 to such investigations can form any idea of the close and pa- 

 tient attention which they require, — the constant watching 

 to obtain the animal in the exact condition or position, in 

 which alone certain peculiarities of structure are apparent ; 

 and, finally, the mortification of finding the conclusions to 

 which we had arrived on one day, after hours of painful at- 

 tention, invalidated by some more favourable observation on 

 the next. 



" In the present communication I have confined myself to 

 the muscular system ; on some other occasion 1 may perhaps 



