432 GENEEAL HISTORY OF THE rNFUSORIA. 



its cavity. As Siebold remarks, it gives rise to an appearance singularly like 

 that of a flickering flame. 



'^ I particularly endeavoured to find any appearance of an opening near the 

 \ibratne cilium, but never succeeded, and several times I thought I could 

 distinctly observe that no such aperture existed. Animals that have been 

 kept for some days in a limited amount of water are especially fit for these 

 researches. They seem to become in a manner dropsical ; and the water- 

 vessels partake in the general dilatation. 



" The band which accompanies the vessel appeared to me to consist merely 

 of contractile substance (connecting-tissue), and to serve as a mechanical 

 support to the vessel. It terminates above in a mass of similar substance 

 containing vacuola attached to the upper plate of the trochal disk." 



This account difi'ers from that of Leydig chiefly in the denial of the patent 

 condition of the free ends of the vibratile tags, and consequently of the en- 

 trance of the fluid from the cavity of the body through them into the lateral 

 vessel. It also casts doubt upon coils of the water- vessel in the neck, and 

 upon the presence of a small non- contractile sac at the inferior termination 

 of the lateral vessels, whilst, on the other hand, it represents anastomosing 

 branches between the vessels of opposite sides in the neck. Mr. Huxley's 

 description therefore appears rather to favoui' Mr. Dahymple's hypothesis 

 as to the contractile vesicle, whilst, with respect to the lateral canals, it is 

 suggestive of a glandular excretory function. 



Dr. Carpenter adopts Prof. Huxley's description of the tags, and of the 

 inosculating vessels in the neck. 



Cohn, in his account of Hydatina scyita (Zeitschr. f. Zool. 1855, p. 444), 

 describes two tubes as springing from the thick-waUed, muscular contractile 

 sac, lying on the abdominal surface of the animal, immediately subjacent to 

 the skin, and communicating with the cloaca. These tubes are " respiratory 

 canals; " they have a finelj- granular wall, and advance with more or fewer 

 curvatures and coils towards the head, where they appear to end in straight, 

 blind, pointed ends or in loops, which attach themselves to the skin of the 

 rotary organ. The ''tags," four on either side, afiixed to the canal are 

 triangular in one aspect, and shortly cylindi'ical in another, and supported 

 on short pedicles, through which their cavity becomes continuous with the 

 interior of the canals. The different figure of the tags from different points 

 of view has given rise to the error of their being of two sorts — cylindrical 

 and triangular. In Bracliionus militaris the contractile vesicle is remarkable 

 on account of its very large dimensions. It occupies as much as two-thirds 

 of the abdominal cavity, and is composed of two chambers, of which the 

 posterior is the larger. The diastole and systole of the two chambers are 

 alternate ; the posterior opens into the cloaca, through a small duct. That 

 there is a direct communication between the contractile sac and the cloaca, 

 Cohn decisively proved by mingling colouiing matter in the water, and wit- 

 nessing a current inwards dui'ing each dilatation, and one outwards on each 

 act of contraction, alternately — an experiment sufficiently conclusive of the 

 respiratory natui^e of the sac. 



The mechanism under consideration appears, as Leydig also remarks, to be 

 occasionally absent — or perhaps only imperceptible. Dr. Dotie states that 

 in FJasculaina " no trace of a vascular system can be obsei-ved. The tremulous 

 giU-hke organs found in some Rotifers are here absent." After his complete 

 examination of Melicerta ting ens, Prof. Williamson says — " I have found no 

 special organs of circulation or respiration .... I detect no vessels or pulsating 

 organs." Nevertheless a structui-e at least resembling the \dbratile tags was 

 noticed in this animal by Mr. Gosse, who states that between the gizzard 



