56 



ZOOGRAPHY. 



1. 



History and Bibliography. 



We have already seen that in the year 1741, Trembley discovered his " Polype a Panache." 

 This is the iirst recorded discovery of a fresh-water Polyzoon. Trembley communicated his 

 discovery to Reaumur and Bernard de Jussieu ; and these celebrated naturalists detected the 

 statoblasts, which they took for eggs, and observed the escape from them of the young 

 Polyzoon. 



In 1744, Trembley published his famous memoirs on fresh-water polypes.* In these is 

 a full description of the " Polype a Panache," accompanied with two figures ; the description is 

 wonderfully accurate, and the anatomical details have been in few points surpassed by subse- 



for examination, and I have been thus enabled to make the annexed drawing, which represents accu- 

 rately the structure so far as it was possible to determine it from the single specimen examined. 



The animal inhabited a fissure in a fragment of rock, from which it was capable of being pro- 

 truded to the length of about two lines, and into which it would again suddenly withdraw on the 

 least annoyance. When fully protruded, it presented the form of a cyliudrical stem, totally destitute 

 of annulation, and bearing upon its summit a crescentic disc, margined with tentacula. This tenta- 

 culiferous disc exactly resembled the lophophore and tentacula of a hippocrepian Polyzoon. In 

 the body of the crescentic disc was placed the mouth, over which there arched a valve-like lip, 

 situated exactly as the epistome of a Polyzoon. The mouth led into an oesophagus which could 

 be easily traced backwards within the perfectly transparent walls of the stem, until it was lost in 

 the piece of rock which concealed the posterior part of the animal. Parallel to the oesophagus, and on 

 the side corresponding to the concavity of the crescent, the rectum was seen passing forwards to open 

 by a distinct anus, in the bottom of this concavity, just beside the mouth. Round the entire margin 

 of the crescent was borne, in uninterrupted succession, a series of ciliated tentacula, surrounded at 

 their base b}' a membrane resembling the calyx of a phylactolsematous Polyzoon. 



So far the structure is absolutely uiidistinguishable from that of a hippocrepian Polyzoon, which it 

 resembles even in minute and apparently non-essential details ; but when we come to examine the rest 

 of the organization, we find that the polyzoal type is widely departed from. On the rectal side of the 

 animal may be seen passing from behind forwards, and in close relation with the intestine, a very 

 distinct pulsating vessel, which carries a stream of red corpusculated blood into the cavity of the 

 crescentic disc. From this the blood passes into all the tentacles, and may be seen as a continuous 

 stream flowing towards their extremities. When it arrives there, it takes a retrograde course, flowing 

 back again through the same channel into the cavity of the disc. We now find that the returning 

 blood has entered another great vessel, which lies u])on the oesophagus, and is formed b}' the uuion of 

 two branches, one from each arm of the crescent, which embrace the oesophagus just behind the mouth. 

 Through this vessel the blood flows backwards in a continuous stream. The great afferent and effereut 

 trunks, now described, were traced as far back as the fissure in which the little animal was lodged, 

 where the concealment of the parts rendered it impossible to follow thera further. In the afferent 



* TuicMBLEY, 'Memoires pour servir h I'Histoirc d'un genre de Polypes d'eau douce.' Leyde, 1744. 



