288 Dr. G. J. Allman on the Homology of the Organs 



terior of the body presents from behind forward foiir cavities, more or less dis- 

 tinguishable from each other, and which there is no difficulty in recognising as 

 the future intestine, stomach, oesophagus, and respiratory sac. As yet, how- 

 ever, there is no trace of longitudinal or transverse bars in the respiratory sac, 

 and it is only at a subsequent period that these bars come to line its walls. 

 Observations are here deficient, but so far as they go it would seem that the 

 transverse bars first make their appearance, that the longitudinal then show 

 themselves ; and lastly, that the sac becomes pierced by the respiratory stig- 

 mata. The circumstances under which the minute tentacula within the orifice 

 of the respiratory sac become developed have not yet been satisfactorily ob- 

 served. So many difiiculties oppose themselves to our observation of the de- 

 velopment of the ovum in the Foli/zoa^ that no facts of importance in the deter- 

 mination of the present question can thence be derived; but if we examine the 

 corresponding development of the hud of Paittrficefc, we shall find after a time, 

 that the nascent Polyzoon presents three distinct cavities, which are to become 

 intestine, stomach, and oesophagus, and which are manifestly homologous with 

 the cavities to which we give the same names iu the embryo-Ascidian. Instead, 

 however, of the closed cavity which in the Ascidian Hes anterior to the cesopha 

 gus, and is to constitute the respiratory sac, we have here the anterior extremity 

 of the oesophagus surrounded by a ring — tlie future lophophore — round whose 

 outer margin a number of minute tubercles soon show themselves, and these 

 then, becoming elongated, constitute the tentacula of the Polyzoon. Now be- 

 tween the formation of these tentacula and that of the respiratory bars of the 

 Ascidue, the resemblance appears quite complete ; in Paludkella and most other 

 Polyzoa^ there is, it is true, nothing homologous with the proper membrane of 

 the respiratory sac of the Amdke (the caliciform membrane of i^rerf^'iceZ^a and 

 the hippocrepian Po^y~oa being here absent), and consequently the closed pree- 

 buccal chamber of \hQAscidice does not exist in them ; but the essential part of 

 the respiratory apparatus — the transverse bars of the Ascidian and the tentacula 

 of the Polyzoon — entirely correspond in their order and mode of development, 

 and so far the evidence derived from the phenomena of development coincides 

 with that afforded by anatomy. In FredericeUa and the hippocrepian Polyzoa, 

 the proper membrane of the sac shows itself in the form of a delicate calyx, which 

 surrounds the base of the tentacular plume ; the difficulty of observing the deve- 



