Morphology of Organs. 163 



dorsal vessel. From this it is not difficult to derive the heart 

 of Astacns. The heart in this animal is continued in front into 

 the ophthalmic artery, and behind into the abdominal ; it has 

 thus quite the appearance of a local thickening of a continuous 

 dorsal trunk, comparable to the dorsal vessel on the earthworm. 

 But the heart of the crayfish is complicated by the presence of 

 an auricle, completely surrounding it, into which open the 

 branchial veins. These may possibly be regarded as the 

 equivalents of the circum-oesophageal "hearts" of the earth- 

 worm, like which, they show distinct traces of metamerism. 

 A further stage is seen in anodon. The ventricle of the heart 

 can be looked upon as a thickening of a dorsal vessel, the 

 arteries behind and before which arise from it representing 

 again a non-modified part of an originally simple dorsal vessel. 

 The two auricles opening into the single ventricle would, in 

 that case, perhaps, be comparable to a single pair of the 

 circum-cesophageal vessels of the earthworm, the segmentation 

 of this part of the vascular system being entirely lost, as is that 

 of all the other organs of the body. 



The same remarks apply to the snail ; but here not only is 

 the segmentation lost, but bilateral symmetry also. 



These invertebrate types differ from the vertebrate in that 

 the heart is dorsal. It is an important morphological difference 

 that in the vertebrate the heart is ventral. Thus, it is probable 

 that the heart in the two series of types is not strictly comparable. 

 Otherwise the concentrated heart of the vertebrate, with its 

 three to four separated cavities, is derivable again from a 

 specialized part of a longitudinal ventral vessel. The heart 

 at first arises as a simple tube, which afterwards becomes 

 twisted. The auricles, therefore, of the frog's heart, derived 

 from the division of an, at first single, auricle, cannot be 

 directly compared with the auricles of anodon; for in the 

 frog they are morphologically the posterior part of the heart- 

 tube, which by subsequent twisting come to lie*in front of the 

 ventricle. Even in the vertebrate it is possible that the 

 prevalent circular vessels of lower types are to be recognized 

 in the aortic arches, which are essentially, in the embryo chick, 

 communications between a dorsal and a ventral vessel. 



