420 EDGAR J. ALLEN. 



at precisely the same time, the two cases diflFering, however, in 

 all probability, in the fact that in the former the cells which 

 subsequently form the walls of the cavity have been through a 

 wandering stage (mesenchyme), whilst those of the latter, so 

 far as we can judge, have not. This does not, however, I 

 believe, alter the conclusions which have been arrived at by 

 the comparison with Peripatus, and which are also supported 

 by what is known of corresponding parts in Arachnids and 

 insects. 



The question suggests itself, what effect has the presence of 

 this persistent coelom in the anterior part of the thorax upon 

 our views as to the nature of the heart of these animals ? The 

 view held by Claus, and which is generally accepted, is that 

 the short heart of Decapods has been reduced from a heart 

 similar to that of Branchipus, which extends throughout 

 almost the whole length of the body. Claus (No. 3) further 

 considers that the Decapod heart represents the anterior por- 

 tion of the heart of Stomatopods, and the evidence which he 

 brings forward in support of this view is very convincing. 

 How are we to account for the fact that in the region of the 

 body anterior to the heart of these Decapods a more primitive 

 condition of things persists than is found even in the adult 

 Peripatus? There appear to me to be two alternatives: 

 either the heart and pericardium have at one time been 

 differentiated along the whole length of the body, aud there 

 has been subsequent degeneration in the anterior portion to 

 the more primitive condition, or we must assume that in the 

 ancestor of the Crustacea the differentiation into heart and 

 pericardium had never taken place at all in this anterior 

 region. What evidence I have been able to gather appears on 

 the whole to favour the latter view, although it is by no means 

 conclusive, and the question can only be settled by further 

 research. The following facts, however, have a bearing on the 

 matter. In Branchipus the cephalic aorta is present, it has 

 no ostia, and does not pulsate. It extends through the region 

 of the maxillae to the second segment of the body (Claus, No. 

 6, p. 71). In the larva of Artemia the heart ends anteriorly 



