ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 743 



is commonly ascribed to them, after a study of their characters in 

 general. 



Notwithstanding the numerous diflferences presented by different 

 groxips, in the successive series of degradations which are to be 

 detected, it is possible to show that the gastric skeleton is never 

 modified except by change in the form or relations of its parts, by 

 their coalescence and disappearance ; nothing new is ever added, and 

 even when most degraded, the homology of the different parts can be 

 asserted almost with certainty. In other words, the gastric skeleton 

 of all the podophthalmate Crustacea is constructed on the same plan. 



The author makes some remarks from a systematic point of view, 

 and urges that the importance of the characters of the stomach is to 

 be exjDlained by the fact that it is not directly subjected to external 

 influences, and is much less exposed to the changes which result from 

 adaptations to environment than are the external organs. These 

 characters then, are of great value in determining the relationships of 

 genera with a different external appearance. 



Mocquard describes the arrangement of the muscles which move 

 the various ossicles, and facilitates the comprehension of his descrip- 

 tion by his figures ; he finds that, at the moment when the gastric 

 muscles contract the median tooth moves forwards, and the anterior 

 ends of the lateral teeth approach one another ; when the gastric 

 muscles relax the apparatus is brought back to its position of equi- 

 librium, by the elasticity of its articulations, and by the action of the 

 cardio-pyloric muscle. When the gastric apparatus is acting, the 

 food tends to be driven upwards, but its passage is prevented by the 

 projections on the urocardiac ossicle. The medio-inferior tooth, 

 though with the form, has not the function of a tooth ; its conforma- 

 tion has no relation to that of the median tooth, and one of the two 

 may vary in form, without the other doing so likewise. 



The author agrees with Cuvier in thinking that the gastric muscles 

 are voluntary in nature. 



The concluding chapter deals with the stomatogastric nervous 

 system, the knowledge of the distribution of which is thought to 

 throw a new light on its physiological activity. While some authors, 

 such as Meckel and J. Muller,have compared it to the great sympathetic 

 of vertebrates, others, such as Newport and Blanchard, have compared 

 it to the pneumogastric nerve. The latter view can only be justified 

 by showing that the stomatogastric system presides over the functions 

 of general sensibility and involuntary movement ; as a matter of fact, 

 however, in the Crustacea, the fibres that pass to the muscles of the 

 tesophagus and labrum are clearly voluntary, and the same is almost 

 certainly true of the branches that go to the motor muscles of the 

 gastric apparatus, and possibly also of those that supply the dilatators 

 and constrictors of the stomach ; some of those that go to the labrum 

 Bccm, moreover, to have a gustatory function. It is possible, how- 

 ever, that the different roots of tho stomatogastric have different 

 functions, and that, when united, they form a mixed trunk more com- 

 plex even than that of the vagus after its union with tho internal branch 

 of the spinal. No observations on this point have as yet been made. 



