ASPECTS. 



Determination of Aspects. 



Another important point, which should be settled at the very outset of our anatomical 

 inquiries, is the exact sense in which we are to use the terms employed to indicate the different 

 aspects of a Polyzoon. This is the more necessary as the terms used for this purpose, in the 

 description of the invertebrate animals generally, are frequently employed in the vaguest 

 l^ossible way, the same term being often applied by different authors to very different aspects 

 of the animal. 



In fixing the meaning of the terms anterior and. posterior we may assume the position of 

 the mouth as indicating the region of the animal which is to be designated as anterior, while the 

 posterior region will then be that diametrically opposite. 



In fixing the dorsal and ventral regions greater difficulty is met with. Mr. Huxley, in his 

 very ingenious and philosophic Memoirs on the Homologies of the Mollusca,* rejects the 

 terms dorsal and ventral altogether ; generalising the Molluscan form under the conception of 

 an ideal archetype, and finding the heart occupying one side, and the great nervous centres 

 placed upon the opposite, he gives to the former region the name of " haemal," and to the 

 latter that of " neural," thus applying to the Mollusca the terms already so happily employed 

 by Owen in his designation of the regions of the vertebrate skeleton. 



These terms have the advantage of stating a simple fact, and of thus avoiding the 

 ambiguity which so often attaches to the terms dorsal and ventral. I shall, therefore, 

 willingly adopt them in the present Memoir, and notwithstanding an apparent contradiction in 

 designating as "hEemal" any portion of an animal totally deprived of a blood-vascular 

 system, I shall call that region of a Polyzoon on which the nervous ganglion lies the " neural," 

 and the opposite region, that, namely, which corresponds to the part of an Ascidian which 

 contains the heart, the " hfemal." 



Tabular view of the Orders and Sub-orders of Polyzoa. 



The reader will be further assisted in the anatomical inquiry in which we are now about 

 to be engaged, by having placed before him here the following scheme of the orders and 

 sub-orders under which all the species of Polyzoa, both marine and fresh-water, admit of 

 being arranged : 



'o 



* 'English Cyclopredia,' 1855, article "Mollusca;" and 'Phil. Trans.,' 1853. 



