208 A. HYATT ON THE PARALLELISM BETWEEN THE INDIVIDUAL 



concentration towards the anterior pole of the Brachiopoda ; the anus remains posterior and 

 the oral region is jet more anterior in the Ascidia ; the anus is posterior and the oral re 

 gion opens at the anterior end in the Lamellibranchiata. At this point the revolution of 

 the mouth stops, it having been transferred from the posterior pole of the Polyzoon to the 

 anterior pole of the Lamellibranchiate along the ventral side, and the revolution of the anul 

 may be said to begin This follows the mouth along the ventral side and in the Gastero- 

 poda and Cephalopoda opens m proximity to the latter on that side, thus completing the 

 inversion. r»- uu o we 



What effect old age may have upon the individual, and what may be its correlations 

 with polarities upon such a vast scale, are questions that I am at present unable to an we 

 m a satisfactory manner. aui,wer 



One fact however, is worthy of notice and exceedingly suggestive. The initiatory trans- 

 formations described above are due to cephalic concentration, a retraction of the whole 

 body within the coenoecial or mantle region. This is the permanent position of the Poly 

 zoon at the two extremes of its life, during its old age, and while it is still an embry^ 

 The Baclnopoda have been homologized by M, Morse and myself/ and the Ascidia by 

 Prof Allman with an invagmated Polyzoon on account of this very cephalic concentration 7 

 and what is stil more curious, the Brachiopoda, which most nearly resemble the n" n^ d' 

 individuals of the Polyzoa, are those that betray an old age peculiarity. It is well kno^ 

 that the old individuals are more globular than the adult or young < and the fori IS 

 have the simple dorsal flexure of the intestine and the oral 'disc or ^^SSS 

 mto horse-shoe shaped arms fringed with tentacles, as in the Hypocrepi n Pol'zoa are Ihe 



described by Milne Edwards m the ^^^^^Z^^ 



selves into their cephalic regions, and in this condition are mo°e closely allied Si 



opoda or Ascidia than at any previous time of life Liiacbi- 



There are other polarities which are similarly placed in their respective divisions The 



of the oral region, and the disposition ci JtS^S^^S^^T^ 

 Limacrd* are the poar forms of the Nudibranchiata, the former ending the series of inter 

 mediate shell-covered Puhnonifera, and the latter beginning the series of bell ^ _ 



Branclnfera. The shell-bearing Pulmonifera, and Branchifera°h ve e^Zlf^ZZl 

 are the normal turbinated Gasteropoda. The polar extremes where ah the v , i 



the coniform mantle of the turbinated Gasteropoda are J*£^£,^'& 

 foot have a much more embryonic appearance than the intermediate norms Z^l 

 to the observations of Lereboullet upon the development of Limneus bwS^Z r 

 observable at a very early age, before the internal organs, with the except L ft r 

 mentary sac, are defined, or the spiral tender,™ nF +h /i exce P tl0n of the ali- 



whole yolk, in fact, is firs't only pail o a creeW so ^ teZ^T^ j, T? *? 

 d. are only creeping discs, wiJthe viscera in placed ^^T^^Z 



1 Milne Edwards. Annates des Sci. Nat. Tom. vi. 1836 3 P m f* ii m nr 



» Hyatt. Observations on Polyzoa. Proc. Esse, Inst 4 , ,, "' ^'^ ^'- ^™, Lond.,1856. 



Vols. iv. and y. 1865-6 ] ^ 00 ^- ^cent and Fossil Shells, p. 213. 



Nat lit? % Emb - y01 - dU LimDde d6S fitaD S s - A ™- ** 

 Mat., 4° bdr. Tom. xvm. 1862. 



