334 C. ROUSSELET ON PEROPHORA LISTERI. 



stomach, and intestine lie properly outside the peritoneum and 

 body cavity of the animal, just as the alimentary canal of 

 vertebrates is suspended in a fold of the peritoneum, but lies 

 outside of it. 



At the posterior part of the body, and opposite the excretory 

 aperture is situated the heart, which is an elongated sac with- 

 out valves, open at both ends, and contracting with a spiral 

 motion. The blood, consisting of a clear fluid with numerous 

 colourless corpuscles, is driven through the blood-channels and 

 spaces between the tissues, the animal having no proper vessels 

 with walls of their own; two main sinuses are situated on 

 opposite sides of the pharynx. The heart, which is enclosed in 

 a pericardium, has this strange peculiarity, that after pulsating 

 a few minutes and propelling the blood in one direction, it 

 stops, and then reverses the action, and the blood flows in the 

 opposite direction. The creeping stalk connecting the various 

 individuals of a group contains two distinct canals, which send 

 branches in each peduncle, and are connected with the heart 

 and circulatory system respectively of each animal, so that the 

 blood of each colony is common property, and circulates through 

 all the members ; yet when one individual is detached by its 

 peduncle it does not die ; the severed canals close up, and the 

 returning current of blood quickly finds its way back into the 

 heart, thus showing that each member is quite independent of 

 the others. 



The circulation of the blood through the heart and through- 

 out the body can be seen to perfection in this species, and can 

 be watched with the microscope under considerable amplifica- 

 tion. 



The nervous system consists of a single ganglion — all that 

 remains from the elongated neural cord of the embryo — placed 

 near the mouth between the two apertures of the body. It can 

 best be seen when looking through the oral orifice in a favour- 

 able position of the animal. The endostyle is a rod-like body 

 lying on the heemal or ventral side of the branchial sac. In 

 Prof. Huxley's words, " It is a longitudinal fold or diverticulum 

 of the middle of the haemal wall of the pharynx, which pro- 

 jects as a vertical ridge into the haemal sinus, but remains in 

 free communication with the pharynx by a cleft upon its neural 

 side." The real nature and function of this structure, which is 



