152 Introduction to Animal Morphology, 



this rotates on its long axis, and soon develops a 

 mouth, stomach, and anus, and Miillerian swellings, 

 as well as a calcigerous gland posteriorly. This body- 

 then becomes barrel-shaped, opaque ; its mouth dis- 

 appears, and it becomes a " Pupa ;" draws in its side 

 lobes ; the stone-canal sinks into the body, the cilia 

 disappear, the tentacles and mouth form, and the 

 animal assumes its mature shape. The power of re- 

 producing lost parts is great. 



As commensal entozoa in Holothurids have been 

 found fishes (Fierasfer), Crustacea (Pinnotheres), and 

 gasteropods (Entoconcha mirabilis, whose eggs and 

 embryos have been found in the ovaries of Holo- 

 thuria), Helicosepina, a shell-less gasteropod, has 

 been found in Synapta digitata. Eulima, Montacuta, 

 and Stylifer, are found outside or inside Echinoids, 

 Starfishes, or Holothurids. 



The class is divided into three orders, containing about 

 270 species. 



1. Apneumona {Brandt) — with no tree-like water-lungs 

 nor Cuvierian organs ; anus and mouth polar ; ambulacral 

 canals 5 ; hermaphrodite. Two families are included : — 

 I. Synaptidae — footless; in Synapta the skin bristles with 

 anchor-like spicules, and millet-seed plates in the intervals, 

 especially along the muscle-bands. Some species (S. in- 

 hserens) have true nettle-cells ; others in which these cells 

 have not yet been demonstrated, have a stinging power. 

 Anapta has no anchors, but biscuit-shaped spines. Chirodota 

 has calcareous wheels, with 6-16 spokes, which may be 

 mounted on an axis (Myriotrochus). Eupyrgus has conical 

 perforated plates and fifteen undivided tentacles. 2. Oncino- 

 labidae — ambulacra present ; tentacles thread-like ; skin with 

 barbed spicules. Echinosoma has no ampulla to the ten- 

 tacles ; no feet, and rudimentary gills. 



2. Tetrapneumona (A/i/^w/v/iz) — with four gill-caeca at the 

 end of the intestine ; mouth and anus at the same end, each 



