OBSERVATIONS ON POLYZOA. 157 



tion of similarity in function as a criterion for the 

 decision of homologies. 



o 



The development of the Ascidia has been deemed 

 closely analogous to the development of the Polyzoa, and 

 the separation of the walls of the branchial cavity from 

 those of the surrounding thoracic chamber in Ascidia has 

 been compared by Prof. Allman with the growth of the 

 lophophore in Polyzoa. 



The recent investigations of Fritz Miiller show, that 

 at one period of its life the Brachiopod has four arms or 

 tentacles arranged in a radiatory manner about the mouth. 

 These parts are borne upon a retractile proboscis, which 

 has some resemblance to the evaginable tube of a Poly- 

 zoon.* 



His researches, however, did not extend to the earliest- 

 periods of growth, and it remains to be seen how far 

 these resemblances are matters of affinity or analogy, and 

 to what degree the young of the Brachiopod really re- 

 peats the peculiarities of the adult Polyzooid. 



EXPLANATION OF TABLE. 



The three systems, as they are presented in the synop- 

 sis, may be read in two ways, either with regard to the 

 mutual connection of organs in the individual, or with 

 regard to their serial relations in the group. Thus by 

 reading from the top to the bottom of any one column, a 

 view of the structure both of the genus and of the indi- 

 vidual is obtainable, the organs being described in regu- 

 lar order, according to their place in the body, from the 

 anterior to the posterior pole. Or, by reading from left 

 to right in the usual manner, an organ or a system may 

 be traced through its entire series of changes in the differ- 

 ent genera, until we reach Cristatella. 



This table was made at the commencement of my in- 

 vestigations, with no intention of publishing it until a 



* Fritz Miiller, Archiv. fiir Anat. Reichart et Du Bois Raymond, 

 p. 72, 1860. 



