51G MR J. D. MACDONALD ON THE MORPHOLOGICAL RELATIONSHIPS 



the mouth of the polyzoon. It may be assumed that the prominent mouth of the 

 Hydrozoon (in which, in truth, there is no stomach homologous with that of the 

 higher types) is equivalent to the everted internal gastric opening of the Acti- 

 nozoon; or, conversely, that the stomach of the Actinia is but an inversion of the 

 oral projection of Hydra, still preserving a communication with the somatic cavity, 

 but necessitating the formation of a new oral orifice. In the same way, the ten- 

 tacula encircling the mouth, in some of the lower forms of animal life, are not in 

 all instances homologous organs. Thus, the branchial tentacula, as they occur in 

 the Ascidiozoa, are found in none of the other members of the series now under 

 consideration ; and in the passage from the tubularian Polyp to the Actinozoon, 

 the oral tentacula of the former (with a single exception, so far as known to me) 

 are suppressed in the latter ; while the outer or somatic set remains, and even 

 becomes more numerous or densely crowded as a general rule. 



The true nature of the Aggregate Tunicata was first made known by Savigny. 

 and they were with great propriety removed from the zoophytes, with which they 

 had been formerly confounded. To M. Milne-Edwaeds is due the credit of 

 having elevated the Polyzoa from their low estate, and ranked them with the 

 Tunicata in his Molluscoid group. Professor Huxley again, from his compre- 

 hensive view of the subject, saw the propriety of associating the Brachiopoda 

 with the Molluscoida, but more immediately with the Polyzoa, as exhibiting the 

 "neural" intestinal flexure, in connection with many very striking points of 

 homology which had never before been conceived.* 



The delicate membrane surrounding the base of the tentacula in the Polyzoa 

 Hijypocrejna is considered by Professor Allman as analogous to the membrane 

 of the respiratory sac in Tunicata ; but Mr Busk says that this has not yet been 

 detected in any marine Polyzoon, though I may add that it is distinctly present 

 in the Brachiopoda ; and he further considers that the membrane surrounding the 

 base of the tentacula in Pedicellina is not homologous with it, having an entirely 

 different import. If ever a Polyzoon resembled a Tunicary, it is the said Pedi- 

 cellina, more especially when its two dorsal bends are in course of development, 

 and a zealous observer might be very readily deceived as to the true nature of 

 certain parts in one bearing a striking but delusive resemblance to those in the 

 other. Yet, without going farther into the refinement of the subject, it would 

 not be far wrong to assume, in round terms, that the pharyngeal respiratory 

 system of the Tunicata is represented by the oral tentacula of the Polyzoa.] 



The epistome of the Polyzoon, moreover, is regarded by Professor Allman as 

 homologous with the languet of the Tunicata. 



* It will be seen, in tlie course of this paper, that I have availed myself of the useful terms 

 employed by Professor Huxley, more particularly with reference to the Calenterata. 



] See Professor Allman''s remarks on this subject, in his valuable work on the fresh-water 

 Polyzoa. Published by the Ray Society. 



