52 HOMOLOGIES. 



From what has now been said, it will perhaps be admitted that the only homological 

 view deducible from the comparison which we have endeavoured to institute between the 

 two great sections of the Molluscoida, is that insisted on in the present memoir. But no view 

 of the homological relations between two sets of organs can be accepted, unless it can also 

 be shown that the conclusions derived from cuudomy are confirmed by the hiatorii of dpvelopmcnt, 

 or at least unless it appears that this history presents nothing irreconcileable with the con- 

 clusions which rest on anatomical evidence alone. 



Unfortunately, the difficulties which beset this part of the subject render us still deficient 

 in the class of facts here needed. We owe to Krohn* some of the most recent investigations 

 into the development of the Ascidife, and, so far as these go, I believe them to be in accordance 

 with the views advanced in the present memoir. In the account given by Krohn, some 

 observations are still wanting before we can determine the exact nature and sequence of all 

 the steps in the development of the Ascidian embryo. From this account, however, it appears 

 that a cavity, which represents that of the future respiratory sac, shows itself at a very early 

 period in the interior of the embryo, having immediately behind it the first indication of the 

 alimentary canal as a tube of uniform diameter bent into a loop. The walls of this cavity are 

 at first in close apposition with those of the thoracic chamber, which completely encloses it, 

 so that the two cavities at this stage appear as one, being exactly co-extensive ; we soon 

 perceive, however, that the proper branchial sac becomes perforated by four apertures, the 

 first appearance of the branchial stigmata, and at the same time becomes detached, in the 

 region of these apertures, from the walls of the thoracic chamber, which had previously itself 

 been perforated b)^ the respiratory and cloacal orifices (the latter at first double, but after- 

 wards single by coalescence). The branchial stigmata subsequently increase in number, the 

 branchial sac at the same time detaching itself more and more from the walls of the thoracic 

 chamber, while the space thus produced, and bounded internally by the walls of the branchial 

 sac and externally by those of the thoracic chamber, becomes the cloacal chamber of the 

 adult. In the intervals of the branchial stigmata, the transverse and longitudinal respiratory 

 bars make their appearance, but the mode in which they originate is not evident ; nor have 

 we any account of the mode of formation of the branchial or thoracic sinus, the first indication 

 of their existence being the current of blood which is seen to flow through them after the 

 heart has begun to act. The alimentary canal, as has just been said, had already shown 

 itself. It has now become more developed, and presents a division into the three regions of 

 oesophagus, stomach, and intestine ; while the mouth opens into the bottom of the branchial 

 sac, and the opposite extremity is found in the cloacal chamber, or space formed by the 

 separation from one another of the walls of the branchial sac and thoracic chamber. 



We have here, then, a series of facts which admit of an easy comparison, so far as they 

 go, with the corresponding steps in the development of the Polyzoa. It will be recollected 

 that, in the development of the bud of a Polyzoon, a minute anterior cavity, closed on all 

 sides, is the first evidence of differentiation. From the walls of this cavity the lophophore 

 subsequently detaches itself, and then gives rise to the tentacles which gradually develop 

 tiiemselves from its margins. The cavity is here to become that of the tentacular sheath, 



* Krnlin in Miillev's ' Arcliiv,' 1852-3; translated in the new series of Taylor's 'Scientific 

 Memoirs,' August, 1853. 



