152 the apodous hoi.othurtans 



Embryology. 



Nothing whatever is known of the embryology of the Molpadiidne, save that 

 Gerould ('96) has studied the oogenesis (and to some extent the spermato- 

 genesis) of Caudina arenata. The mature ovum is about 200 /^ in diameter, 

 and has a conspicuous and ])eculiar micropyle. The mature spermatozoan is 

 about ()() iJ- long, with the head about '.].6 /^ in diameter. 



PhY'SIOLOGY'. 



Almost nothing is yet known of the vital activities of the Molpadiidfp, for 

 few zoologists have ever had the opportunity of carrying on extended observa- 

 tions on living specimens. Sluiter ( '88) has kept specimens of Aphelodactyla 

 punctata alive in aquaria, and Gerould ('9G) has had similar success with Cam- 

 dhta areiiafa, but neither has published any extensive account of the physiology 

 or habits of these species. 



Motion. — The movements of the Molpadiida^ are accomplished by muscular 

 contraction, aided by the fluids in the tentacles and body-cavity. It is possible 

 that in some cases (such as in young Molpadise) the calcareous particles may 

 assist; l)ut certainly, in many cases, the deposits of the body-wall jilay no part. 

 Movement forward is possible only when the animal is buried in the sand or 

 mud. If placed on the surface, the first movements are downward, and continue 

 until the animal is buried. The movement, either downward or forward, is 

 accomplished partly by swallowing the sand or mud immediately in front of 

 the oral disc, but chiefly by backward and forward radial movements of the 

 tentacles ; these movements are effected by the alternate contraction of the 

 longitudinal muscles of their outer and inner sides. The contractions of the 

 muscles of the body-wall do not appear to play any important part in progressive 

 movements, but are actively concerned in respiration {q. v.). The rate of pro- 

 gressive movement is exceedingly slow, perhaps averaging about 1 mm. per 

 minute. 



Digestion and absorption. — The food passing into the stomach doubtless 

 undergoes the first stages of digestion there; but, if we may infer anything 

 from the distribution of the blood-vessels, the chief activity of the alimentary 

 canal is in the small intestine. In that region and the first part of the large 

 intestine probably all the absorption occurs, but nothing is really known as to 

 the physiology of the digestive system. 



Circulation and nutrition. — Here again we have to infer from structure, 

 for we know nothing definitely. The extensive haemal system would seem to 

 indicate that the absorbed food material is transported by it to all parts of the 

 body, but there is obviously no true circulation of the fluid, the plasma simply 

 moving outward into the tissues, bearing the essential food. The indigestible 



