1'ROF. ASAS'SIZ'S 



and there woui'd be no d'oiibt left as to the final 

 circumscription of that group. 



But there are animals which mast be referred to 

 the type of Mollusks, according to our present 

 knowledge of their structure, which differ so wide- 

 ly in their appearance from Mollusks, that, at first, 

 when mentioned, this combination seems utterly 

 unnatural and unfounded and indeed, leaving the 

 impression as if there could be no foundation for 

 a natural system, if such combinations were to be 

 considered as natural Nevertheless, I think that 

 the association of some animals which I am aboat 

 to illustrate, will be found to rest on real aMnity ; 

 and that the external differences in form will have 

 no influence upon the impression which such a 

 combination will leave. 



[;See Plate XXXIX, page S3.} 

 We have here in Plate XXXIX, and in several oth- 

 er diagrams [which the Professor exhibited to the 

 audience,] Polype-like animals, resembling Folypi 

 very much by their stems, with cells ia which there 

 are living animals extending and contracting in a 

 manner similar to Polypi, with tentacles around 

 their mouths, which at in a manner resem- 

 bling Polypi still more than the stem in -v?hich they 

 are included. And these animals do not belong to 

 the type of Polypi ; : they are true Mollasks. The 

 discovery of their internal structure was made 

 almost simultaneously by Ehrenbrg, by Milne-Ed- 

 . wards, and by Mr. Thompson, of Cork r so that their 

 relation to Mollasks is now known to be very 

 close. They have a relation to the radiated type 

 of Polypi by the fringes around the mouth. 

 But the arrangement of their whole system is truly 

 bilateral. 



This ngure (Plate XXXIX, fig. C)' represents the 

 alimentary canal, which differs very much from 

 the Radiata, ia being curved upon itself, in having 

 distinct openings, a large sac which represents the 

 stomach, and a structure which comes very near 

 that of some animate which have never been sep- 

 arated from Mollusks. If we were only to consid- 

 er those, perhaps the resemblance to Mollusks 

 might have escaped observation. 



But let me now trace further than I did before 

 the analogies which exist between Mollasks. I 

 compared the Gasteropoda with the Acephala and 

 the Cephalopoda; I showed that there was one 

 type in the bivalves, ia the univalves and in 

 Cephalopoda. But between the Ascidias (Plate 

 XLVII), and the Clams, (Plate XXXV), there 

 are only slight differences. Suppose the shell of 

 the Clam (Plate XXXV) to disappear, the mantle 

 to be almost entirely removed, the respiratory 

 tube to be shortened, and the two openings to be 

 somewhat remote, and we shall have such an an- 

 imal as is represented in Flate XLVII, fig. H, en- 

 closed ia a sac with two openings, which'are not 

 the openings of the alimentary canal, but are the 

 openings which lead into a cavity ia which all the 

 rgans are coatained. 



Plate X%XV, page 78.} 



And BOW going further, we may have all possi- 

 ble modifications of this type whea it is contracted 

 and when the peduncle is attached. Plate XLVII; 

 f g. A, represents a fixed Ascidia, the peduncle be- 

 ing only a prolongation of the sac-like envelope. 

 Here we have two openings of the sac, correspond- 

 ing to ths two openings of the clamshell Eeyond 

 this type, we may have oae ia which several indi- 

 viduals ere u-aited by their base. And then, from- 

 single animals, we pass to compound animals 

 combined by their attachment on one spot, (Plat 

 XLVII, fig. F) or by a gelatinous envelope which 

 keeps the eggs together, (Tig. B), and constitutes' 

 compound animals. 



The interaal structure of these Ascidia, (Plate 

 XLVII, fi'g. C.} : , is so like that of Clams, that there 

 is ao difficulty about their analogy. Now, one 

 step further, and suppose that the gelatinous en- 

 velope which unites these individuals seeretes- 

 calcareous substance. Suppose further,, that each 

 individual is much smaller, and ia addition,, 

 that one extremity, instead of presenting fringes 

 at its opening, is- surrounded by threads ; theis 

 you have the structure of the Bryosoa, (in Plate 

 XLTIIIK with' a calcareous stem, with a sym- 

 metrical alimentary canal, but with serrated tenta- 

 cles round its anterior aperture, coastituting a pe- 

 type th$ Bryosoa. And tliaS t&ey 



