172 NOTES AND MEMORANDA. 



tissues of the Medusaa has the eflfect of maintaining the make and 

 break stimulations at their maximum value ; but the value of these 

 stimulations raj^idly declines if they are successively repeated with 

 the current passing in the same direction. 



In the sub-umbrella of the MedusaB waves of nervous excitation 

 are sometimes able to pass when waves of muscular contraction have 

 become blocked by the severity of overlapping sections. 



J^xhaustion of the sub-umbrella tissues — esi)ecially in narrow con- 

 necting isthmuses of tissue — may have the effect of blocking the 

 passage of contractile waves, 



Lithocysts have been proved sometimes to exert their ganglionic 

 influence at comparatively great distances from their own seats — ^con- 

 tractile waves originating at points in the sub-umbrella tissue remote 

 from a lithocyst, and ceasing to originate at that j^oint when the 

 lithocyst is removed. A nervous connection of tliis kind may be 

 maintained between a lithocyst and the point at which the waves of 

 contraction originate even after severe forms of section have been inter- 

 j)osed between the lithocyst and that point. 



When the sub-umbrella tissue of Aurelia is cut throughout its 

 whole diameter, the incision will again heal up, sufficiently to restore 

 physiological continuity, in from four to eight hours. 



Tetrapteron volitans. — This peculiar marine hydrozoon was im- 

 perfectly described in 1851 by M. Busch, who named it Tetraplatia 

 volitans. It has now been re-discovered by Professor C. Claus, who 

 gives it the name at the head of this paragraph.* 



The animal in the extended condition is of an elongated pear- 

 shape, but four-sided instead of circular in section ; the smaller end 

 bears the oral aperture, and answers to the manubrium of a medusa, 

 the larger or aboral end answering to the bell. At the middle (that is, 

 half-way between the oral and aboral poles) of each of the four faces is 

 a depression, from which springs a bilobed wing-like appendage, pro- 

 vided with muscles, by the flapping of which the animal is propelled 

 through the water, with the aboral pole forwards. In each division 

 of each wing is an otolithic sac. The mouth leads into an enteric 

 cavity, which is continued into the aboral portion of the hydrosoma. 

 Reproductive organs occur as four masses, probably ectodermal pro- 

 ducts, in the four longitudinal edges of the body. 



The ectoderm consists of large ciliated cells, some of which con- 

 tain thread-cells, while in others the protoplasm is so modified as to 

 form a gland, presenting a distinct aperture, and a radiating arrange- 

 ment of the glandular contents. The endoderm cells are so extensively 

 vacuolated as to form a mere network of plasma-threads. The vacuoles 

 probably contain the albuminoid products of digestion ; in some of 

 them small aggregations of crystalline rods are found, probably the 

 final products of urinary metabolism. Amongst these vacuolated cells 

 smaller granular endoderm cells occm- at intervals, two or three 

 together. 



Between the ectoderm and endoderm is a structureless connective 



* ' Aivhiv f. Mikr. Anat.,' vol. xv. (1878) p. 8i9. 



