CRISTATELLA MUCEDO. 67 



Bearing in mind, then, that Cristatella is placed in the 

 '^ Folyzoa" class of the "Molluscoida" division, it will be well 

 perhaps to draw attention first to the points of difference between 

 the Polyzoa and the other two classes. The true position of 

 the Brachiopoda, which are inclosed in a bivalve shell, seems at 

 present to be doubtful. In the later researches of Professor 

 Allman, as embodied in his monograph of fresh-water Polyzoa, he 

 does not agree with Huxley in considering that certain similarities 

 of structure with the Polyzoa are of sufficient importance to 

 include them in that group, as they have a much nearer relation- 

 ship to the true Mollusca. But between the Twiicata and the 

 Polyzoa there is considerable resemblance, for the Tufiicata 

 (which includes the Ascidians and the Salpse), like the Polyzoa, 

 inhabit a double-walled sac, yet there are two great distinctions 

 between these classes, ist. — The Polyzoa are entirely destitute of 

 heart and blood vessels, the fluid within the perivisceral cavity 

 being clear and colourless, and being kept constantly in motion 

 by internal cilia ; while the Tunicata, on the other hand, have a 

 distinct though imperfect heart, and the sac-walls are lined with a 

 complete system of blood-vessels. This heart, or circulatory 

 apparatus, differs from that of all other animals in that it is 

 entirely destitute of valves, being a simple tube, beating with 

 measured pulsations that drive the blood first through one end 

 and then through the other, so reversing the currents alternately. 

 2nd. — The Polyzoa are distinguished from the Tunicata, inas- 

 much as the alimentary canal can be protruded and withdrawn by 

 the processes of evagination and invagination. The mouth, with 

 its ciliated tentacula, rises from the body of the creature and unfolds 

 itself in one of the most beautiful forms of animal inflorescence. 

 Whether it be Plumatella or Cristatella, nothing can exceed the 

 beauty of the long, waving, silver-like tentacles, with their rapidly 

 vibrating, almost indistinguishable, cilia, especially when viewed 

 under a spot-lens with good illuminating power. The Tunicata, 

 on the contrary, have no means of protrusion, their movements 

 being wholly carried on within the sac-walls. These two points of 

 variation are well shown in the accompanying diagrams (PI. VIII., 

 Figs. 3 — 5), which has been copied from a drawing in Professor 

 AUman's well-known monograph on the Polyzoa. In Fig. 3, the 



