CRUSTACEA. 525 



(and so far we do not know about other forms), become divided 

 into somites, at any rate with the same distinctness that is usual 

 in Annelids and Arthropods. Not only so, but there is-at-first 

 no marked division into a somatic and splanchnic layer with an 

 intervening body cavity. Some of the cells become differentiated 

 into the muscles of the body wall and limbs ; and other cells, 

 usually in the form of a very thin layer, into the muscles of the 

 alimentary tract. In the tail of Palcemon Bobretzky noticed 

 that the cells about to form the muscles of the body were 

 imperfectly divided into cubical masses corresponding with the 

 segments ; which however, in the absence of a central cavity, 

 differed from typical mesoblastic somites. In Mysis Metschni- 

 koff states that the mesoblast becomes broken up into distinct 

 somites. Further investigations on this subject are required. 

 The body cavity has the form of irregular blood sinuses amongst 

 the internal organs. 



Heart. The origin and development of the heart and vascular system 

 are but very imperfectly known. 



In Phyllopods (Branchipus) Claus (No. 454) has shewn that the heart is 

 formed by the coalescence of the lateral parts of the mesoblast of the ventral 

 plates. The chambers are formed successively as the segments to which 

 they belong are established, and the anterior chambers are in full activity 

 while the posterior are not yet formed. 



In Astacus and Palaemon, Bobretzky finds that at the stage before the 

 heart definitely appears there may be seen a solid mass of mesoblast cells 

 in the position which it eventually occupies 1 ; and considers it probable that 

 the heart originates from this mass. At the time when the heart can first 

 be made out and before it has begun to beat, it has the form of an oval sack 

 with delicate walls separated from the mesenteron by a layer of splanchnic 

 mesoblast. Its cavity is filled with a peculiar plasma which also fills up the 

 various cavities in the mesoblast. Around it a pericardial sack is soon 

 formed, and the walls of the heart become greatly thickened. Four bands 

 pass off from the heart, two dorsalwards which become fixed to the 

 integument, and two ventralwards. There is also a median band of cells 

 connecting the heart with the dorsal integument. The main arteries arise 

 as direct prolongations of the heart. Dohrn's observations on Asellus 

 greatly strengthen the view that the heart originates from a solid meso- 

 blastic mass, in that he was able to observe the hollowing out of the mass in 



1 Reichenbach describes these cells, and states that there is a thickening of the 

 epiblast adjoining them. In one place he states that the heart arises from this thicken- 

 ing of epiblast, and in another that it arises from the mesoblast. An epiblastic origin 

 of the heart is extremely improbable. 



