88 COMPARATIVE ANATOMY. 



namely : the capacity for general development possessed by 

 animals of a given type. In Radiata where this is small, 

 the eye or ear are never seen to attain that degree of com- 

 plexity secured by Articulata or Mollusca. 



LASSO CELLS. 



Affixed to tentacles of Physalm (Portuguese man-of-war), 

 Cyanea, JEucoryne, Hydra, Astrangia, Actinia, etc. are peculiar 

 cell-like structures, endowed with the property of exciting, 

 on contact with a sentient surface, a prickling or stinging 

 sensation. 



The essential feature to a stinging apparatus is a filiferous 

 capsule. This, within an oval or ovoidat cell, contains a 

 spirally wound thread, or consists of smaller cells, each con- 

 taining, in addition, a style extending from one pole to about 

 the centre of the cell.* 



ORGANS OF TOUCH. 



PROTOZOA. Parts specially designed for exercising the 

 sense of touch do not exist, if the long lash-like appendages 

 (flagella) to certain Infusoria be not excepted. 



RADIATA. Excluding the lasso cells, no organs of touch 

 are met with in Co3lenterata. The tentacles are in part sub- 

 servient to this purpose. 



* Under circumstances not readily explained, the cells eject their contained 

 threads with an astonishing degree of rapidity ; in so doing the cells are ab- 

 solutely turned inside out, and remain attached to the emptied cells as long 

 extended tubes. From the smaller cells the style is also extruded, and then 

 appears as a more expanded portion of the thread, with which it is contin- 

 uous at one end and the capsule at the other. An attentive examination of 

 the extruded thread exhibits a more complicated structure than would have 

 been suspected, and its exact character is extremely difficult to ascertain. 

 In the case of the larger capsules a spiral arrangement is readily distin- 

 guishable, extending the entire length of the extruded thread. This arrange- 

 ment, in some instances, appeared to depend upon minute cilia which project 

 at right angles from the thread and apparently pursue a spiral course ; but 

 in other instances it appears as if the thread, during its eversion of the 

 capsule, assumed a spiral course within the portion preceding it, and that 

 the thread externally was encircled at regular intervals with non-vibrating 

 cilia. (Leidy.) 



