84 ARTHUR E. SHIPLEV. 



The Vascular System. 



There are two varieties of blood-corpuscle found in Phynio- 

 soma. The larger kind exist in great numbers in the body- 

 cavity, together with the ripe generative products (fig. 30). 

 They are oval, about "02 mm. long and two-thirds as broad ; 

 their protoplasm is very clear and transparent, but the nucleus 

 stains well and they have a very definite outline. The coelomic 

 fluid, in which these corpuscles float, bathes all the internal 

 organs of the animal, and when the contraction of the poste- 

 rior circular muscles forces the fluid forward it would serve to 

 evert the introvert, which is withdrawn again by the retractor 

 muscles. 



The second variety of blood-corpuscle is much smaller than 

 the first, being about half as long and as broad ; the proto- 

 plasm is not so transparent and stains more readily. These 

 corpuscles are contained in a close space which is usually called 

 the vascular system. This space may best be described as 

 consisting of three parts, all communicating with one another. 

 The first of these is a horseshoe-shaped space (figs. 2 and 7) 

 at the base of the tentacles. From this space there runs up 

 into each tentacle a series of three vesseJs which anastomose 

 freely with each other and communicate at the tip. As a rule 

 sections of the tentacles show one vessel near the inner pig- 

 mented surface of the tentacle, just external to the tentacular 

 nerve and two near the outer surface, one each side of the 

 ciliated groove (fig. 17). The free ends of this horseshoe- 

 shaped space at the base of the tentacles, near the dorsal 

 middle line, are continuous with the ends of another horseshoe- 

 shaped space which lies in the collar. This forms the second 

 of the above-mentioned spaces. As the diagram (fig. 2) shows, 

 it is very irregular in form, breaking up and anastomosing 

 into a number of spaces. This communicates only with the 

 inner smaller horseshoe, between the two is the crescentiform 

 space in which the mouth opens. The third space — usually 

 termed the dorsal blood-vessel — is a very extensile sac running 

 along the dorsal middle line of the oesophagus between the 



