292 LIONEL JAMBS PICTON. 



droplets as colourless. They seem to me to bear the same 

 colour relatiou to the yellow colour of the cell-body as the 

 blood-corpuscles of Audouiiiia bear to the colouring matter 

 dissolved in the plasma of the latter worm. 



Development of the Heart-body in Polymnia nebu- 

 losa. — The work of Saleusky, who described the primitive 

 cavity in the heart-body of Terebella Meckelii, and the 

 communication between this and the coelom ; and also Eisig's 

 conjecture based on that observation^ that the organ is due to 

 an infolding of the heart wall, have already been mentioned. 



Although much discussion has taken place on the question 

 as to whether the heart-body be a hypoblastic or a mesoblastic 

 organ, no further embryological work has been published on 

 the point. 



In Polymnia nebulosa^ the matter can be settled by an 

 examination of transverse and longitudinal sections of the 

 heart- body shortly after its first appearance in the larva. 



As in Terebella Meckelii, the blood-system is earliest 

 represented by a peri-intestinal blood-sinus ; but whereas in 

 that species the dorsal and ventral vessels made their 

 appearance only in a larva of twenty segments, in Polymnia, 

 at a stage when nine pairs of chsetse were counted, a ventral 

 vessel was clearly defined ; whilst in a larva of thirteen pairs 

 of chtetae a dorsal vessel also had appeared in the anterior part 

 of the body. But as yet there was no heart-body. There were, 

 however, cells in the vessel walls, which were swollen, and con- 

 tained collections of greenish-yellow pigment (fig. 33). The 

 blood-pigment which gradually makes its appearance is pro- 

 bably derived from this. 



The exact moment at which the heart-body forms is diflScult 

 to determine. In a larva about 1*5 cm. in length it has 



* The eggs of Polymnia are obtainable in great numbers at Naples, and 

 develop well in captivity. A.lso large numbers of the larvae are found amongst 

 the algae from certain beds. The tube is very early secreted, and is removed 

 with great difficulty. Cav. Dr. Lo Bianco, however, told me that, on 

 allowing them to stand for a few hours in stagnant sea water, the larvae came 

 out of tlieir own accord. By this means they may be obtained living and 

 without their tube, with great ease. 



