22 MEMOIR II. 



These, and all the other magnified sketches, with whieh 

 it is intended to illustrate the smaller subjects in the 

 progress of these memoirs, will tend to show, how absurd 

 it is to think of communicating a clear idea of almost any 

 minute animal without them, and how idle to rest satisfied 

 with representations of the natural size, since so much 

 that is worthy to be seen and admired, and which appears 

 necessary to the right understanding of their true nature 

 remains invisible to the unaided sight. 



In these little animals when young, and particularly in 

 Mysis Vulgaris, the circulation can be better observed 

 than perhaps in any other of the Crustacea. The Heart, 

 which is situated under the centre of the corselet behind, 

 is of an elongated form, (Plate I. fig. 15, b ;) at its fore 

 part it gives off an anterior aorta, which going towards 

 the head is speedily veiled from the sight by the opacity of 

 the matters contained in the stomach (a) and intestine, 

 over which it runs ; at its opposite end in like manner it 

 furnishes a posterior aorta, (c) which may be traced to the 

 end of the tail ; at each side, it further appears to receive 

 a vessel of smaller size, which is probably the united trunks 

 of the pulmonary veins, reconveying the aerated blood 

 fi'om the branchia, again to go the round of the circulation ; 

 the pulsations of the heart are so rapid as to resemble 

 vibrations, and together with the blood it is so transparent 

 and colourless, that but for the globules of the latter we 

 should hardly be able to trace the course of the circulation, 

 and which in the figure is represented by little arrows. 

 On observing with attention the termination of the poste- 

 rior aorta at the end of the tail, a periodical action may 

 be noticed, as if of the opening and closing of a valvular 

 opening on each side, accompanied each time by the filling 

 of the corresponding end of a vessel of considerable size, 

 lying on each side of the intestinal canal, ((/) these vessels 

 or veins, propel the blood towards the heart by successive 



