262 A. S. PEARSE. 



ten dendroid tentacles which may be extended for feeding or col- 

 lapsed and completely concealed by the turning in of the anterior 

 end of the body. Just behind the tentacles is a "collar" which 

 is without the slender tube-feet which cover the rest of the body. 

 The tube-feet on the ventral and lateral surfaces have well devel- 

 oped sucking discs at their tips but such adhesive organs are not 

 uniformly present on the dorsal side of the body. Besides this 

 difference in the tube-feet, the dorsal and ventral surfaces may be 

 distinguished from each other by the fact that the former is 

 shorter than the latter and hence the ends of the body turn up- 

 ward when the animal rests on its ventral side. They may be 

 further distinguished by the difference in color (ventral being 

 lighter), by the presence of a single median dorsal genital papilla 

 between the bases of the tentacles, and by the fact that the two 

 ventral tentacles are much shorter than the others. The internal 

 organs are strikingly bilateral in their arrangement. This is 

 shown by the right and left respiratory trees which unite and have 

 a common opening on the dorsal side of the cloaca, by the single 

 dorsal gonad and madreporite, and by the ventral Polian vesicles. 

 The organs of the body wall are, on the other hand, typically 

 radial in arrangement and hence the skeletal, muscular, water- 

 vascular and nervous systems conform in general to the usual 

 echinoderm type of structuije. The water-vascular and nervous 

 systems both consist of a circum-oral ring from which five radial 

 branches extend in the body wall to the region of the cloaca. 

 The skeletal parts consist of ten ossicles around the mouth which 

 together form "the lantern," and of minute calcareous plates im- 

 bedded in the skin at the anterior and posterior ends of the body. 

 The muscular system consists of five longitudinal bands, which 

 extend from the circum-oral ossicles to the posterior end of the 

 body, and of circular muscles which lie just beneath the skin and 

 completely fill the space between the longitudinal muscles. The 

 digestive tube is considerably coiled and is differentiated into four 

 regions : an oesophagus which passes through the lantern, an ex- 

 panded stomach, a slender intestine, and a musular cloacal 

 chamber which gives rise dorsally to the respiratory trees. 



From this brief synopsis of the structure of TJiyone, it will be 

 seen that, though we are dealing with an animal having a radiate 



