ON THF- DIPLOCHOUDA. 311 



upon the presence of transverse lines due to the contraction. 

 He compared the structure of the muscle-cells to that of those 

 lying over tiie '' diverticulum/' 



Blood System. — The vascular system has received a great 

 deal of attention by former observers. Wagener's first obser- 

 vations were corrected in some particulars by Schneider. The 

 latter found a dorsal vessel on the stomach with indications of 

 its double nature. He also described a number of blind sacs 

 which, as finger-like processes, project into the ccelom at the 

 junction of stomach and intestine. Their presence had been 

 noticed prior to this by Lenckart and Pagenstecher. 



Schneider, along with some others, states that the blood- 

 corpuscles arise from the masses of cells referred to above in 

 connection with the collar nephridia. 



Metschnikoff (14) states the vascular system to be in direct 

 communication with the body cavity, apparently as an inference 

 from the fact that these masses of cells, at first in the body 

 cavity, are found later in the blood-sinuses. Krohn, again, 

 gives a description of the dorsal and ventral trunks and 

 the csecal vessels, he observed the contraction and expan- 

 sion of the cyecal vessels; and Wilson (22) figures the main 

 features of the vascular system as determined by his prede- 

 cessors. 



Caldwell (2) confirmed the results of his own work and that 

 of previous workers as follows: — "1. Blood-corpuscles aggre- 

 gated in two or more masses, lying free in the body cavity of 

 the pre-oral lobe, i. e. in front of the septum. 2. A blood- 

 vessel formed on the dorsal wall of the stomach — a marked 

 structure in the larva. 3. The splanchnopleure sac, which in 

 the region of the stomach forms a loose sac surrounding the 

 gut. 4. Csecal prolongations of this sac. 5. Csecal prolonga- 

 tions into the rudiuients of the adult tentacles. '' 



I have been enabled not only to confirm some of the above 

 observations, but to complete the account of the blood system. 

 It consists essentially of a system of sinuses and fissures lying 

 outside the mesoblast and between it and the epiblast or hypo- 

 blast. These may be regarded as vestiges of the segmentation 



