HISTORY. 57 



quent observers. Trembley has demonstrated a complete alimentary canal, consisting of 

 oesophagus, stomach, and intestine ; but though he has seen the emission of the excrementitial 

 matter from the intestine, the actual termination of this tube has escaped him. He has observed 

 the retractor muscles, and has noticed the funiculus attached to the fundus of the stomach ; 

 this last organ, however, he supposes to be a muscle destined, like the true retractors, for the 

 withdrawal of the polypide into its cell. He has shown the connection between the polypide and 

 its cell, and demonstrated that the former ought not to be viewed as the mere fabricator of the 

 latter, as the caddis worm is of his case, but that each is an inseparable part of one and the 

 same animal. He witnessed also the circulation of the fluid in the perigastric space. This 

 circulation was rendered evident by the presence of small spherical bodies, which were kept 

 in continual motion, and were often propelled from one cell into a neighbouring one ; a fact 

 which at once led him to infer the continuity of the cavities of the different cells. These cor- 

 puscles he suspected, but without sufficient grounds, to be the eggs of his polype. We find 

 that, in a letter addressed to Bonnet, and quoted by this philosopher in his ' Considerations sur 

 les Corps Organises,'* Trembley describes with great accuracy the statoblasts of a fresh-water 



vessel the current of blood was intermittent. Immediately on being emptied, the whole of the visible 

 portion of the vessel contracted vigorously, and then slowly expanded for the reception of a fresh influx 

 of blood, the pulses being at very regular intervals. No reversed action could be observed in the 

 pulsation of this vessel, though, as already mentioned, the current of blood in the tentacula was 

 alternately direct and inverse. In the efferent vessel there was no pulsation, and the current was 

 here perfectly continuous. The circulation in the interior of the cresceutic disc appears to be properly 

 extra-vascular, and the cavity of this disc must be viewed as a great shms. 



Another important point of departure from polyzoal structure is found in the absence of a proper 

 perigastric space, the alimentary canal and vessels being fixed in the stem, and embraced by its walls, 

 so as to leave little or no intervening space, while the perigastric fluid is here replaced by the true 

 blood in its proper vessels. There is, therefore, no power of invagination, and retraction is probably 

 an act entirely similar to that by which the ordinary tubicolous Annelides are witlidrawn into their 

 tubes. Well-marked longitudinal, muscular (?) fibres are visible in the stem. 



The nervous system was not detected ; and, as it was impossible to expose the posterior part of 

 the animal without running the risk of its entire destruction, much of its organization still remains 

 altogether unknown. 



As to the true relations of the little animal now described, it appears to me that they are to be 

 found with the Annelides rather than with any other group of the animal kingdom ; and, notwitli- 

 standing the singular resemblance in certain parts of its structure between it and a liippocrepian 

 Polyzooii, I believe that this resemblance points to no real afliuity, and must be viewed only as a 

 remarkable example of representative form, — of homomorpMsm as distinguished both from lioraology 

 and analogy, — of which the resemblance between the Polyzoa and tlie Polypes affords another, and 

 which is par.alleled in the vegetable kingdom by such instances as that presented by the repetition 

 of the Cactus form in the African Euphorbise. 



The above account was drawn up and ni the printer's hands before I had read Dr. Wright's 

 description, from which it slightlj' differs in one or two particulars, but in none which can render 

 nugatory the special point which has caused its introduction here ; name!}', the remarkable and highly 

 instructive example afforded by it of the assumption of polyzoal form by a widely diftcrent type of 

 organization, without this identity of form in any way indicating a relation of homology, 



* Bonnet, 'Considerations sur les Corps Organises.' Amst., 1762. 



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