OF CONCHOLOGY. 



47 



The most extraordinary manner in which some Ascidians are 

 developed, as worked out by Kovalevsky and others, would, in a 

 strictly embryological classification, place them in the vertebrata. 

 On the other hand, other nearly allied species are without the 

 singular features referred to, in their embryonic condition. This 

 may be referred to the suppression of certain stages by some 

 naturalists ; yet, the very admission that suppression of char- 

 acters in the embryonic progress is possible, undermines the 

 whole value of embryology as an assistant in systematic classi- 

 fication ; for who shall say when or to what extent suppression 

 may not have occurred and what essential features of embryonic 

 life may not have been lost thus in any given case ? 



It has been stated that the flexure of the intestine in Appen- 

 dicularia is radically different from that of the other tunicaries, 

 but this does not appear by the later researches of Moss.* 

 On the other hand, Mr. Alder describes certain species of As- 

 cidice (A. parallelogramma, and PhaUusia turcica) as having the 

 intestine flexed in a different direction from that which usually 

 obtains. These facts are further proofs of the little dependence 

 to be placed upon this character as a basis for classification. 



For our present purpose it is unnecessary to define the Tuni- 

 cata and Polyzoa, and for the sake of brevity I shall confine my- 

 self to giving a definition of the class Brachiopoda. Attention 

 should be directed, however, to the homologies between the 

 Polyzoa and Conchifera, as suggested by Rhabdopleura and 

 worked out by that well-known student of the Polyzoa, Mr. 

 Allman.f The homologies between tunicates and conchifers, 

 drawn by Allman and Hancock, masters of the subject, are well 

 known. 



Class BRACHIOPODA. 



Animals provided with a shelly covering composed of two 

 valves, each of which is normally bilaterally symmetrical, and 

 to which it is organically attached by three principal pairs of 

 muscles. Soft parts also bilaterally symmetrical ; consisting 

 essentially of a mantle composed of two lobes, to which the 

 valves correspond, of which lobes the outer edges are disunited 

 throughout the greater part, or the whole of their extent ; a disk 

 of membrane, variously modified in form, with its edges fringed 

 with a series of tubular brachia ; the mouth situated within this 

 disk at its posterior portion ; a stomach with a more or less an- 

 teriorly recurved intestine ; a circulatory system, more or less 



* Trans. Lin. Soc. Vol. xxvii, Part ii, p. 299, pi. xlvii, 1870. 

 f Journal of Microscopical Science, N. S. ix, p. 63, 1869. 



