366 Zoological Classification. 



but no heart ; " the matters which result from digestion perco- 

 lating through the walls of the intestines, and becoming 

 mixed with the perivisceral fluid." It is remarkable that in 

 these creatures the intestine is curved like the letter U, so 

 that the anus is bent round to approach the gullet. The 

 nervous ganglion is placed between the two legs of this 

 flexure, aud a bend of this kind is called " neural." 



The Polyzoa are all compound or associated animals, 

 whence their name ; but when a polyzoon egg is hatched, as in 

 the case of the Plumatella described in our vol. ii. p. 271, it 

 commences life as an isolated being, and by a subsequent 

 growth, resembling budding, multiplies into a colony. 



The Brachiopoda (lamp shells), which bear considerable 

 resemblance in structure to the Polyzoa, are, like the higher 

 animals, not capable of this colonial multiplication — their only 

 offspring living as single creatures, like their parents. " In 

 this family the intestine," says Professor Huxley, " either 

 ends blindly in the median line, or else terminates in a distinct 

 anus." A similar statement might be made concerning the 

 Rotifers, as while a distinct anus is usual, in the Asplanchna the 

 digestive cavity terminates in a cut de sac. The Brachiopoda, 

 like the Ascidians, possess what is called an " atrial system/' 

 or set of canals distinct from the true vascular system. Pro- 

 fessor Huxley adopts the view that these atrial vessels carry 

 away excretory matters, and the products of the reproductive 

 organs. In the former of these functions they appear to re- 

 semble the water-vascular vessels of the Rotifera, to which 

 excretory, as well as respiratory, functions, may probably be 

 assigned. 



In the Ascidians the atrial system is remarkably deve- 

 loped, and is in free communication with the pharynx, " and 

 as on the margins of the pharyngo-atrial apertures are fringed 

 with cilia, working towards the interior of the body ; a current 

 is produced which sets in at the oral aperture, and out at the 

 atrial opening, and may be readily observed in a living Asci- 

 dian." Small young transparent specimens afford beautiful 

 spectacles with a moderate magnification. In the Ascidians, 

 the first bend of the intestine would carry it away from tho 

 nervous ganglion. This kind of flexure is termed hcemal, 

 and the relative positions of the nervous and blood systems 

 will be found to possess great distinctive importance. 



Wo need not dwell on the Lamellibranchiate animals be- 

 cause they will be better understood by referring the reader 

 to the Rev. Mr. Haughton's paper on the " Swan Mussel," 

 No. xxxii. p. 67. In these creatures a well developed heart 

 is found, and the gills, arranged in " lamelhe," or leaves, 

 furnish beautiful microscopic objects, iu which ciliary action 



