HOMOLOGIES. 51 



view a number of important facts, and has defended it with more ingenuity tiian it has 

 received from any other anatomist.* 



Imagining an ideal molluscan archetype, Mr. Huxley derives from this general plan all 

 the special modifications presented by the molluscan sub-kingdom. From this archetype he 

 thus derives the polyzoal form. " Conceive the abdomen of the archetype to be greatly pro- 

 longed, the neural region with its appendages, the organs of sense, and the heart, remaining 

 undeveloped, so that the anus comes into close apposition with the oral extremity, while the 

 edges of the latter are produced into long ciliated tentacles ; and the result will be a Polyzoon, 

 which needs only the power of gemmation to give rise to those composite aggregations 

 which are so characteristic of the group. "t 



In these words we have an excellent and easily comprehensible enunciation of the 

 essential morphological characters of a Polyzoon. 



In deriving the ascidian form from the archetype, "it is to be remarked," he says, "in 

 the first place, that the pharynx, large in the Polyzoa, becomes comparatively enormous in 

 the Ascidians; while the tentacles, which were very large in tlie Polyzoa, are in the Ascidians 

 comparatively small. Next with the development of a post-abdomen, the intestine acquires 

 a hfemal flexure ; but instead of tiie anal aperture remaining on the haemal side, it is bent 

 round by the same process as in Spinalis and Limacina, but in the inverse direction. Suppose, 

 with all this, that a mantle has been developed, and that its free margin, remaining small and 

 narrow, has followed the anus to the neural side, while its cavity has extended up on each 

 side of the pharynx to the middle line of the haemal surface of the latter, carrying to a great 

 extent a process of which the outline may be seen in C^inbidia, and giving rise to the atrium ; 

 — imagine also that the sac thus constituted externally by the inner surface of the mantle 

 (third tunic) and internally by the pliai'ynx, becomes perforated by minute apertures — and 

 the result would be an Ascidian. "t 



Now, if the above be an exact statement of the true morphology of the Ascidian, it is 

 inconsistent with the views attempted to be demonstrated in the present paper. After a very 

 careful consideration, however, I have not been able to adopt it. Its essential feature 

 consists in the peculiar explanation it offers of the formation of the " atrium," the term given 

 by Mr. Huxley to the space, including the cloaca, which intervenes between the branchial 

 sac and the third tunic of the Ascidise ; and it necessarily involves the supposition that tlie 

 walls of this cavity wrap themselves round the pharynx in the manner of a serous sac, just 

 as the serous lining of the pericardium in the higher animals wraps itself round the heart ; 

 and that the pharyn.x is therefore, properly speaking, external to the cavity of the atrium. 

 I have never succeeded in satisfying myself of the realit}' of this condition, and must still 

 believe that the walls of the atrium simply surround the branchial sac, without being reflected 

 on its sides ; that the branchial sac is, therefore, properly witJiin the cavity of the atrium, just 

 as the mouth and labial palps are within the mantle-cavity of the lamellibranchiate Mollusca, 

 and the tentacular crown of the Polyzoa within the tentacular sheath during the retracted 

 state of the polypide. 



* 'British Association Reports ' for 1852, and 'English Cyclopaedia,' 1855, article "Mollusca." 

 f Article " Mullnsca " iu ' English Cyclopedia, 1855, column 858. 

 \ Id., column 864. 



